ARM: shmobile: marzen-reference: Remove legacy clock support

Marzen DT reference is now only built for multiplatform
which means that CCF comes with the package. Remove unused legacy
code ifdefs to clean up the code.

Based on similar work for the Koelsch board by Magnus Damm.

Cc: Magnus Damm <damm@opensource.se>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>