ARM: vt8500: prepare for arch-wide .init_time callback

Current vt8500 board init calls of_clk_init() from vtwm_clk_init. To allow
consolidation of DT driven .time_init, move of_clock_init() to a temporary
.time_init callback that will be removed when arch-wide callback is available.
With previous pmc_base parsing helper for vt8500 clock providers, we can also
safely remove the call to vtwm_clk_init() and get rid of some includes.

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