ARM: shmobile: Use r8a7791 DT CPU Frequency in common case

Convert the common C-code-less r8a7791 DT board support
to use shmobile_init_delay() to be able to migrate away
from per-SoC delay setup functions.

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