ARM: dts: msm: Add power grid regulator phandles for SM6150

Add secondary phandles to each of the PM640 and PM640L regulator
devices which match the naming scheme used in the power grid
hardware documentation.  This should help to alleviate confusion
between similarly named regulators on different PMICs
(e.g. pm640_l1 and pm640l_l1 vs L1A and L1C). Also add tertiary
phandles for VDD_CX, VDD_MX, and VDD_MMCX supplies which each
serve the same purpose for several consumers.

Change-Id: Ib976a506dcd26efa35c3bec59c39366d83b8ada1
Signed-off-by: Kiran Gunda <kgunda@codeaurora.org>