ARM: shmobile: marzen-reference: Initialize CPG device

On multiplatform kernels clocks are handled by the CCF CPG driver. It
must be explicitly initialized by a call to r8a7779_clocks_init() with
the value of the boot mode pins.

Based on similar work for the Koelsch board by Laurent Pinchart.

Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>