ARM: shmobile: Remove redundant r8a7790 callback

Replace the SoC-specific callback init_machine() with
a NULL to use the default code. This cleans up the code
and reduces the number of lines.

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