drivers: sh: Get rid of CONFIG_ARCH_SHMOBILE_MULTI

Shmobile is all multiplatform these days, so get rid of the reference to
CONFIG_ARCH_SHMOBILE_MULTI.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>