ARM: vexpress: Initial common clock support

This patch makes Versatile Express use the common clock framework
instead of the plat-versatile implementation.

It defines clock provider for VE's OSCs (clock generators) and
registers all required fixed and variable clock sources (for both
motherboard and core tile).

This is a simple conversion of the existing state and will be
extended (and migrated to drivers/clk) in the near future.

Signed-off-by: Pawel Moll <pawel.moll@arm.com>