scsi: dt-bindings: ufs: Add document for ufs-mediatek

Add UFS and UFS PHY node document for Mediatek SoC chips.
Signed-off-by: Stanley Chu <stanley.chu@mediatek.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

Documentation/devicetree/bindings/ufs/ufs-mediatek.txt

+* Mediatek Universal Flash Storage (UFS) Host Controller
+
+UFS nodes are defined to describe on-chip UFS hardware macro.
+Each UFS Host Controller should have its own node.
+
+To bind UFS PHY with UFS host controller, the controller node should
+contain a phandle reference to UFS M-PHY node.
+
+Required properties for UFS nodes:
+- compatible         : Compatible list, contains the following controller:
+                       "mediatek,mt8183-ufshci" for MediaTek UFS host controller
+                       present on MT81xx chipsets.
+- reg                : Address and length of the UFS register set.
+- phys               : phandle to m-phy.
+- clocks             : List of phandle and clock specifier pairs.
+- clock-names        : List of clock input name strings sorted in the same
+                       order as the clocks property. "ufs" is mandatory.
+                       "ufs": ufshci core control clock.
+- freq-table-hz      : Array of <min max> operating frequencies stored in the same
+                       order as the clocks property. If this property is not
+                       defined or a value in the array is "0" then it is assumed
+                       that the frequency is set by the parent clock or a
+                       fixed rate clock source.
+
+Example:
+
+	ufsphy: phy@11fa0000 {
+		...
+	};
+
+	ufshci@11270000 {
+		compatible = "mediatek,mt8183-ufshci";
+		reg = <0 0x11270000 0 0x2300>;
+		interrupts = <GIC_SPI 104 IRQ_TYPE_LEVEL_LOW>;
+		phys = <&ufsphy>;
+
+		clocks = <&infracfg_ao INFRACFG_AO_UFS_CG>;
+		clock-names = "ufs";
+		freq-table-hz = <0 0>;
+	};