ARM: dts: msm: Set a53 cpu retention voltage to 0.55v on MSM8994

Set a53 cpu ldo retention voltage to 0.55v to allow an additional
50mV margin on top of the characterized and recommended value.
This additional margin is to ensure stability on targets that may
fall outside the 50mV margin already in place.

Change-Id: I8d7a387b6e4c8c870f5c0f3ba9b7939ca5d351fd
Signed-off-by: Karthik Parsha <kparsha@codeaurora.org>