clk: qcom: gdsc-regulator: Add MMCX specific logic

When clients call the regulator_is_enabled() API for GDSCs that
are sourced off of the MMCX rail, any register accesses that will
be made as part of the callback rely on the presumption that the
MMCX rail is enabled. When the rail is disabled, the corresponding
ahb clocks that are needed for these registers to be accessible
will also be turned off, thus leading to an unclocked access. Check
to make sure that the MMCX rail is enabled prior to reading or
writing to the MM registers.

Along similar lines, add logic to enable the parent rail for the
GDSC prior to reading and writing to the GDSC registers during the
set_mode and get_mode callbacks.

Change-Id: I8265bc147efd2acd1d6638638eb78935762feee8
Signed-off-by: Deepak Katragadda <dkatraga@codeaurora.org>