dt-bindings: phy: Add `tx-p2p-microvolt` property binding

Common PHYs and network PCSes often have the possibility to specify
peak-to-peak voltage on the differential pair - the default voltage
sometimes needs to be changed for a particular board.

Add properties `tx-p2p-microvolt` and `tx-p2p-microvolt-names` for this
purpose. The second property is needed to specify the mode for the
corresponding voltage in the `tx-p2p-microvolt` property, if the voltage
is to be used only for speficic mode. More voltage-mode pairs can be
specified.

Example usage with only one voltage (it will be used for all supported
PHY modes, the `tx-p2p-microvolt-names` property is not needed in this
case):

  tx-p2p-microvolt = <915000>;

Example usage with voltages for multiple modes:

  tx-p2p-microvolt = <915000>, <1100000>, <1200000>;
  tx-p2p-microvolt-names = "2500base-x", "usb", "pcie";

Add these properties into a separate file phy/transmit-amplitude.yaml,
which should be referenced by any binding that uses it.
Signed-off-by: Marek Behún <kabel@kernel.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/devicetree/bindings/phy/transmit-amplitude.yaml

+# SPDX-License-Identifier: (GPL-2.0 OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/phy/transmit-amplitude.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Common PHY and network PCS transmit amplitude property binding
+
+description:
+  Binding describing the peak-to-peak transmit amplitude for common PHYs
+  and network PCSes.
+
+maintainers:
+  - Marek Behún <kabel@kernel.org>
+
+properties:
+  tx-p2p-microvolt:
+    description:
+      Transmit amplitude voltages in microvolts, peak-to-peak. If this property
+      contains multiple values for various PHY modes, the
+      'tx-p2p-microvolt-names' property must be provided and contain
+      corresponding mode names.
+
+  tx-p2p-microvolt-names:
+    description: |
+      Names of the modes corresponding to voltages in the 'tx-p2p-microvolt'
+      property. Required only if multiple voltages are provided.
+
+      If a value of 'default' is provided, the system should use it for any PHY
+      mode that is otherwise not defined here. If 'default' is not provided, the
+      system should use manufacturer default value.
+    minItems: 1
+    maxItems: 16
+    items:
+      enum:
+        - default
+
+        # ethernet modes
+        - sgmii
+        - qsgmii
+        - xgmii
+        - 1000base-x
+        - 2500base-x
+        - 5gbase-r
+        - rxaui
+        - xaui
+        - 10gbase-kr
+        - usxgmii
+        - 10gbase-r
+        - 25gbase-r
+
+        # PCIe modes
+        - pcie
+        - pcie1
+        - pcie2
+        - pcie3
+        - pcie4
+        - pcie5
+        - pcie6
+
+        # USB modes
+        - usb
+        - usb-ls
+        - usb-fs
+        - usb-hs
+        - usb-ss
+        - usb-ss+
+        - usb-4
+
+        # storage modes
+        - sata
+        - ufs-hs
+        - ufs-hs-a
+        - ufs-hs-b
+
+        # display modes
+        - lvds
+        - dp
+        - dp-rbr
+        - dp-hbr
+        - dp-hbr2
+        - dp-hbr3
+        - dp-uhbr-10
+        - dp-uhbr-13.5
+        - dp-uhbr-20
+
+        # camera modes
+        - mipi-dphy
+        - mipi-dphy-univ
+        - mipi-dphy-v2.5-univ
+
+dependencies:
+  tx-p2p-microvolt-names: [ tx-p2p-microvolt ]
+
+additionalProperties: true
+
+examples:
+  - |
+    phy: phy {
+      #phy-cells = <1>;
+      tx-p2p-microvolt = <915000>, <1100000>, <1200000>;
+      tx-p2p-microvolt-names = "2500base-x", "usb-hs", "usb-ss";
+    };