i2c: shmobile: Use ARCH_SHMOBILE instead of SUPERH

"i2c-sh_mobile" is used on sh7343, sh7366, sh7722, sh7723, and sh7724
only. As all of the above select ARCH_SHMOBILE, restrict its driver
dependencies from SUPERH to ARCH_SHMOBILE.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>