ARM: shmobile: r8a7740 pinmux platform device cleanup

Use DEFINE_RES_MEM() and platform_device_register_simple()
to save a couple of lines of code.

Signed-off-by: Magnus Damm <damm@opensource.se>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>