clk: vt8500: parse pmc_base from clock driver

Currently, clock providers for vt8500 depend on machine_init providing
pmc_base address before calling of_clk_init. With upcoming arch-wide
.time_init calling of_clk_init, we should make clock providers independent
of mach code. This adds a pmc_base parsing helper to current clock provider
that gets called if there is no pmc_base set, yet.

Signed-off-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Acked-by: Tony Prisk <linux@prisktech.co.nz>
Acked-by: Mike Turquette <mturquette@linaro.org>