ARM: shmobile: Remove non-multiplatform Genmai reference support

Now that r7s72100 has CCF support, remove the legacy Genmai reference
Kconfig bits for the non-multiplatform case.

Starting from this commit Genmai board support is always enabled via
CONFIG_MACH_GENMAI, and CONFIG_ARCH_MULTIPLATFORM is used to select
between board-genmai.c and board-genmai-reference.c

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

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>