clk: versatile: Split config options for sp810 and vexpress_osc

Move the Kconfig entry for Versatile (& Express) clock drivers
into a separate file and add individual options for sp810
and vexpress_osc drivers, as they are optional in some
configurations and may have separate dependencies.

Signed-off-by: Pawel Moll <pawel.moll@arm.com>
Acked-by: Mike Turquette <mturquette@linaro.org>