arm/dt: msm8974: Reduce PM8841 SMPS 4 regulator default voltage to 0.815 V

The consumers of the PMIC PM8841 SMPS 4 regulator on MSM8974
targets can correctly operate when S4 outputs 0.815 V.
Therefore, change the regulator-min-microvolt property of the S4
device node to reflect this lower value.  Also change the
qcom,init-voltage property to match the minimum constraint so
that power is saved at runtime.

Change-Id: I5db37d1dfc7882228d2b99b8d07731545be3620f
Signed-off-by: David Collins <collinsd@codeaurora.org>