clk: socfpga: Read the clock parent's register base in probe function

This commit (82de42fa) calls child's
ofdata_to_platdata() method before the parent is probed in dm core.
This has caused the driver no longer able to get the correct parent
clock's register base in the ofdata_to_platdata() method because the
parent clocks will only be probed after the child's ofdata_to_platdata().
To resolve this, the clock parent's register base will only be retrieved
by the child in probe() method instead of ofdata_to_platdata().

Signed-off-by: Chee Hong Ang <chee.hong.ang@intel.com>
Reviewed-by: Ley Foon Tan <ley.foon.tan@intel.com>