clk: shmobile: Rework CONFIG_ARCH_SHMOBILE_MULTI

Shmobile is all multiplatform these days, so get rid of the reference to
CONFIG_ARCH_SHMOBILE_MULTI in drivers/clk/shmobile/.

Also instead of always enabling DIV6 and MSTP adjust the Makefile
to enable DIV6 and MSTP depending on if they are included in the
SoC or not.

Signed-off-by: Magnus Damm <damm+renesas@opensource.se>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>