ARM: dts: msm: enable register retention for GDSC regulators on KONA

Specify the qcom,retain-regs property for KONA gdsc-regulator
devices so that subsystem retention registers associated with
each GDSC maintain their state when a given GDSC is disabled
and then re-enabled.

Change-Id: If4dedf9a89734d17a528db9f4f7c00c48f13354c
Signed-off-by: David Collins <collinsd@codeaurora.org>