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

The external crystal frequency is 20MHz on the Koelsch board. Specify it
in the device tree.

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