dt-bindings: arm: Document Renesas V3MSK and Wheat board part numbers

The DT binding documentation for the Renesas V3MSK and Wheat boards
lacked board part numbers.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
[simon: corrected Wheat part number]
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>