clk: qcom: gdsc: Add support for min operational vote on GDSC parent

This change is squash of below changes-

clk: qcom: gdsc-regulator: Place a min operational vote on GDSC parent

For GDSCs that reside on domains that are collapsible and
support a minimum voltage of retention, explicitly vote on
the minimal operating voltage, LowSVS for the rail. This is
required as an interim step before enabling/disabling the
GDSC. Once that's done and the GDSC has latched on to its
new state, remove this voltage vote.

clk: qcom: gdsc: Fix issue with dereferencing NULL pointer in gdsc APIs

Fix the issue with deferencing the rdev structure for
the GDSC prior to it being populated. Replace the
qcom,vote-parent-supply-voltage property with
vdd_parent-supply and use the regulator handle that's
returned from calling regulator_get for the parent
regulator directly.

Change-Id: Ibac43bba8248d68f8355d1a9db9cfcbfb9fa7557
Signed-off-by: Deepak Katragadda <dkatraga@codeaurora.org>
Signed-off-by: Odelu Kukatla <okukatla@codeaurora.org>