ARM: dts: qcom: Add "qcom,voltage-ramp-step" for PM8150L/PM6150L

Add "qcom,voltage-ramp-step" property to enable the LCDB driver
to ramp-up the voltage in steps of 500 mV for PM6150L/PM8150L.
This is as per the hardware recommendation.

Change-Id: I3b84cded31118e3e57efd4665c4f7e0909b476e3
Signed-off-by: Kiran Gunda <kgunda@codeaurora.org>