ARM: shmobile: Consolidate Legacy SH_CLK_CPG Kconfig

Consolidate SH_CLK_CPG under ARCH_SHMOBILE_LEGACY so it can
be shared by the legacy ARM mach-shmobile support code.

Signed-off-by: Magnus Damm <damm+renesas@opensource.se>
Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>