ARM: shmobile: r8a7779: Use DT CPU Frequency in common case

Convert the common C-code-less r8a7779 DT board support
to use shmobile_init_delay() to be able to migrate away
from per-SoC delay setup functions.

Based on work by Magnus dam for the r8a7740 SoC.

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