ARM: dts: msm: specify VDD_MX as a parent of VDD_MMCX for Kona

Add VDD_MX as a parent for VDD_MMCX so that requests made for
VDD_MMCX are automatically propagated to VDD_MX.  This ensures
that device memories receive sufficient voltage as VDD_MMCX is
scaled up to meet the device logic hardware requirements.

Change-Id: Iaa69bc83a61af2eeaee48dae8216070bc2aed65f