ARM: shmobile: Add platform device tree bindings documentation

Add Renesas SH-Mobile, R-Mobile, and R-Car Platform Device Tree Bindings
Documentation, listing supported SoCs and boards.

This allows to use checkpatch to validate DTSes referring to Renesas
shmobile SoCs, and boards containing those SoCs.
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
[horms+renesas@verge.net.au: tweaked title]
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>

Documentation/devicetree/bindings/arm/shmobile.txt

+Renesas SH-Mobile, R-Mobile, and R-Car Platform Device Tree Bindings
+--------------------------------------------------------------------
+
+SoCs:
+
+  - Emma Mobile EV2
+    compatible = "renesas,emev2"
+  - RZ/A1H (R7S72100)
+    compatible = "renesas,r7s72100"
+  - SH-Mobile AP4 (R8A73720/SH7372)
+    compatible = "renesas,sh7372"
+  - SH-Mobile AG5 (R8A73A00/SH73A0)
+    compatible = "renesas,sh73a0"
+  - R-Mobile APE6 (R8A73A40)
+    compatible = "renesas,r8a73a4"
+  - R-Mobile A1 (R8A77400)
+    compatible = "renesas,r8a7740"
+  - R-Car M1A (R8A77781)
+    compatible = "renesas,r8a7778"
+  - R-Car H1 (R8A77790)
+    compatible = "renesas,r8a7779"
+  - R-Car H2 (R8A77900)
+    compatible = "renesas,r8a7790"
+  - R-Car M2-W (R8A77910)
+    compatible = "renesas,r8a7791"
+  - R-Car V2H (R8A77920)
+    compatible = "renesas,r8a7792"
+  - R-Car M2-N (R8A77930)
+    compatible = "renesas,r8a7793"
+  - R-Car E2 (R8A77940)
+    compatible = "renesas,r8a7794"
+
+
+Boards:
+
+  - Alt
+    compatible = "renesas,alt", "renesas,r8a7794"
+  - APE6-EVM
+    compatible = "renesas,ape6evm", "renesas,r8a73a4"
+  - APE6-EVM - Reference Device Tree Implementation
+    compatible = "renesas,ape6evm-reference", "renesas,r8a73a4"
+  - Atmark Techno Armadillo-800 EVA
+    compatible = "renesas,armadillo800eva"
+  - BOCK-W
+    compatible = "renesas,bockw", "renesas,r8a7778"
+  - BOCK-W - Reference Device Tree Implementation
+    compatible = "renesas,bockw-reference", "renesas,r8a7778"
+  - Genmai (RTK772100BC00000BR)
+    compatible = "renesas,genmai", "renesas,r7s72100"
+  - Gose
+    compatible = "renesas,gose", "renesas,r8a7793"
+  - Henninger
+    compatible = "renesas,henninger", "renesas,r8a7791"
+  - Koelsch (RTP0RC7791SEB00010S)
+    compatible = "renesas,koelsch", "renesas,r8a7791"
+  - KZM9D
+    compatible = "renesas,kzm9d", "renesas,emev2"
+  - KZM-A9-GT
+    compatible = "renesas,kzm9g", "renesas,sh73a0"
+  - KZM-A9-GT - Reference Device Tree Implementation
+    compatible = "renesas,kzm9g-reference", "renesas,sh73a0"
+  - Lager (RTP0RC7790SEB00010S)
+    compatible = "renesas,lager", "renesas,r8a7790"
+  - Mackerel (R0P7372LC0016RL, AP4 EVM 2nd)
+    compatible = "renesas,mackerel"
+  - Marzen
+    compatible = "renesas,marzen", "renesas,r8a7779"
+
+Note: Reference Device Tree Implementations are temporary implementations
+      to ease the migration from platform devices to Device Tree, and are
+      intended to be removed in the future.