ARM: dts: msm: add PON regulator for PMI8994

Add a PON regulator (pon_perph_reg) for controlling bit 1 of
PON_PERPH_RB_SPARE (0x88C) of PMI8994.

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: 840571
Change-Id: I437d6170c2a362af1cd71b21dc90cbd3d0c33cfe
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>