ARM: shmobile: marzen-reference: Use DT CPU Frequency

Convert the Marzen DT reference board support to use shmobile_init_delay()
to be able to migrate away from per-SoC delay setup functions.

Based on work for the Armadillo800 EVA board by Magnus Damm.

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