ARM: shmobile: Remove non-multiplatform Marzen reference support

Now that r8a7779 has CCF support remove the legacy Marzen reference
Kconfig bits CONFIG_MACH_MARZEN_REFERENCE for the non-multiplatform
case.

Starting from this commit Marzen board support is always enabled via
CONFIG_MACH_MARZEN, and CONFIG_ARCH_MULTIPLATFORM is used to select
between board-marzen.c and board-marzen-reference.c

The file board-marzen-reference.c can no longer be used together with
the legacy sh-clk clock framework, instead CCF is used.

Based on work for the Koelsch board by Laurent Pinchart.

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