clk: qcom: gdsc-regulator: Poll PWR_ON during GDSC HW to SW mode switch

While switching from HW to SW control mode, HW may be busy updating
internal required signals. Add a check that polls for PWR_ON status
bit to ensure that the GDSC switches to SW mode before software starts
to use SW mode.

Change-Id: Ie6bd5c5e8c073fb70c696eeb6e911219108f9260
Signed-off-by: Vivek Aknurwar <viveka@codeaurora.org>