Add SPI device for RSPI on Genmai.

On this board, only rspi4 is in use. Its bus contains a single device
(a wm8978 audio codec), for which no bindings are defined yet.
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>