ARM: shmobile: Use r8a7791 DT CPU Frequency for Koelsch

Convert the Koelsch 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>