ARM: dts: Add RSKRZA1 DT bindings documentation

Add RSKRZA1 Device tree bindings Documentation, listing it as a supported
board.

This allows to use checkpatch to validate DTSes referring to the RSKRZA1
board.
Signed-off-by: Chris Brandt <chris.brandt@renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>

Documentation/devicetree/bindings/arm/shmobile.txt

@@ -63,6 +63,8 @@ Boards:
     compatible = "renesas,marzen", "renesas,r8a7779"
   - Porter (M2-LCDP)
     compatible = "renesas,porter", "renesas,r8a7791"
+  - RSKRZA1 (YR0K77210C000BE)
+    compatible = "renesas,rskrza1", "renesas,r7s72100"
   - Salvator-X (RTP0RC7795SIPB0010S)
     compatible = "renesas,salvator-x", "renesas,r8a7795";
   - Salvator-X