ARM: shmobile: marzen: Specify external clock frequency in DT

The external crystal frequency is 31.25 on the Marzen board.
Specify it in the device tree.

Based on work for the Lager board by Laurent Pinchart.

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