arm: shmobile: Change platform dependency to ARCH_RENESAS

Since commit 9b5ba0df ("ARM: shmobile: Introduce ARCH_RENESAS")
ARCH_RENESAS is a more appropriate platform dependency than the legacy
ARCH_SHMOBILE, hence use the former.

This will allow to drop ARCH_SHMOBILE on ARM in the near future.

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