clk: vt8500: Remove platform dependant initialization

pmc_base can be initialized by searching the devicetree and finding the
parent node which must be the Power Management Controller. This allows
us to remove vtwm_clk_init(), and arch/arm/mach-vt8500/common.h.

Update the binding document to clarify that clock nodes should be children
of their PMC node.

Signed-off-by: Tony Prisk <linux@prisktech.co.nz>