ARM: dts: msm: add PON regulator for PMI8950/PMI8937

Add PON regulator (pon_perph_reg) for controlling bit 1 of
PON_PERPH_RB_SPARE (0x88C) of PMI8950/PMI8937.

This regulator can be enabled by the consumers whenever they
want to vote for the MBG to operate in normal power mode (NPM)
without which the MBG can enter low power mode (LPM) during PMIC
sleep (i.e. XO shutdown) causing performance degradation.

CRs-Fixed: 1003346
Change-Id: I0f8e0d38a3016c93f06183c1f775ab4ae4260ba7
Signed-off-by: Abinaya P <abinayap@codeaurora.org>