clk: qcom: gdsc-regulator: add support for enabling retention registers

Add support for the RETAIN_FF_ENABLE feature which enables the
usage of retention registers.  These registers maintain their
state after disabling and re-enabling a GDSC.

Change-Id: Iad19bfb2cb73fa76aad230bad6372d68b447b4e2
Signed-off-by: David Collins <collinsd@codeaurora.org>