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

Current socfpga board init calls of_clk_init() from .machine_init. To
allow consolidation of DT driven .time_init, move of_clock_init() to
a temporary .time_init that will be removed when arch-wide callback is
available.

Signed-off-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Acked-by: Dinh Nguyen <dinguyen@altera.com>