clk: sprd: support to get regmap from parent node

Some SC9863a clock nodes would be the child of a syscon node, clocks can
use the regmap of syscon device directly for this kind of cases.

Signed-off-by: Chunyan Zhang <chunyan.zhang@unisoc.com>
Link: https://lkml.kernel.org/r/20200304072730.9193-7-zhang.lyra@gmail.com


Signed-off-by: Stephen Boyd <sboyd@kernel.org>