ARM: dts: msm: Update LDO2 regulator-min-voltage to 500mV for QM215

Update LDO2 regulator-min-voltage to the value lower than
retention voltage (600mV) to keep the MX rail in retention
mode during the sleep.

Also remove the qcom,init-voltage as the regulator framework
sends the min-voltage during the regulator_register.

Change-Id: I99fde7f8cd50e6b90d8ee56a03a0c3e819560cac
Signed-off-by: Kavya Nunna <knunna@codeaurora.org>