ARM: shmobile: Introduce r8a7791_add_standard_devices()

Introduce the function r8a7791_add_standard_devices() that
follows the same style as other mach-shmobile SoC code and
allows C version of board code to add on-chip devices.

Signed-off-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>