ARM: shmobile: Remove non-multiplatform Lager reference support

Now that r8a7790 has CCF support remove the legacy Lager reference
Kconfig bits CONFIG_MACH_LAGER_REFERENCE for the non-multiplatform
case.

Starting from this commit Lager board support is always enabled via
CONFIG_MACH_LAGER, and CONFIG_ARCH_MULTIPLATFORM is used to select
between board-lager.c and board-lager-reference.c

The file board-lager-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>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>