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>