ARM: shmobile: Remove non-multiplatform Koelsch reference support

Now that r8a7791 has CCF support remove the legacy Koelsch reference
Kconfig bits CONFIG_MACH_KOELSCH_REFERENCE for the non-multiplatform
case.

Starting from this commit Koelsch board support is always enabled via
CONFIG_MACH_KOELSCH, and CONFIG_ARCH_MULTIPLATFORM is used to select
between board-koelsch.c and board-koelsch-reference.c

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

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
[horms+renesas@verge.net.au: Dropped arch/arm/boot/dts/Makefile portion]
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>