dt-bindings: mtd: partitions: add TP-Link SafeLoader layout

Most TP-Link home routers use the same partitioning system based on a
custom ASCII table.

It doesn't seem to have any official name. GPL sources contain tool
named simply "make_flash" and Makefile target "FlashMaker".

This partitions table format was first found in devices with a custom
SafeLoader bootloader so it was called SafeLoader by a community. Later
it was ported to other bootloaders but it seems the name sticked.

Add binding for describing flashes with SafeLoader partitions table. It
allows operating systems to parse it properly and register proper flash
layout.
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20221015092950.27467-1-zajec5@gmail.com

Documentation/devicetree/bindings/mtd/partitions/tplink,safeloader-partitions.yaml

+# SPDX-License-Identifier: GPL-2.0-only OR BSD-2-Clause
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/mtd/partitions/tplink,safeloader-partitions.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: TP-Link SafeLoader partitions
+
+description: |
+  TP-Link home routers store various data on flash (e.g. bootloader,
+  flash layout, firmware, product info, configuration, calibration
+  data). That requires flash partitioning.
+
+  Flash space layout of TP-Link devices is stored on flash itself using
+  a custom ASCII-based format. That format was first found in TP-Link
+  devices with a custom SafeLoader bootloader. Later it was adapted to
+  CFE and U-Boot bootloaders.
+
+  Partitions specified in partitions table cover whole flash space. Some
+  contain static data that shouldn't get modified (device's MAC or WiFi
+  calibration data). Others are semi-static (like kernel). Finally some
+  partitions contain fully changeable content (like rootfs).
+
+  This binding describes partitioning method and defines offset of ASCII
+  based partitions table. That offset is picked at manufacturing process
+  and doesn't change.
+
+maintainers:
+  - Rafał Miłecki <rafal@milecki.pl>
+
+properties:
+  compatible:
+    const: tplink,safeloader-partitions
+
+  partitions-table-offset:
+    description: Flash offset of partitions table
+    $ref: /schemas/types.yaml#/definitions/uint32
+
+required:
+  - partitions-table-offset
+
+additionalProperties: false
+
+examples:
+  - |
+    partitions {
+        compatible = "tplink,safeloader-partitions";
+        partitions-table-offset = <0x100000>;
+    };