dt-bindings: regulator: gdsc-regulator: add property for register retention

Add the description for a new property, qcom,retain-regs, which
can be used to specify that retention registers should be
configured to maintain their state after a GDSC is disabled
and re-enabled.

Change-Id: Ic3649de0804d47117bddff72f3713c1ab24e33b2
Signed-off-by: David Collins <collinsd@codeaurora.org>