ARM: shmobile: genmai legacy: Add RSPI support

Add RSPI platform device, resources, platform data, and SPI child.

On this board, only rspi4 is in use. Its bus contains a single device
(a wm8978 audio codec).

Signed-off-by: Geert Uytterhoeven <geert+renesas@linux-m68k.org>
Acked-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>