ARM: dts: msm: make VDD_CX dependent upon VDD_MX for sdm845

Specify the active + sleep and active-only VDD_MX regulators as
the parents of the corresponding VDD_CX regulators.  Also specify
a minimum dropout voltage level of -1.  This will ensure that
VDD_CX requests automatically generate equivalent VDD_MX requests
since voltage levels are sparsely defined and get rounded up.

This is used to enforce a requirement that exists for various
hardware blocks that VDD_MX voltage level >= VDD_CX voltage level
for a given operating frequency.

Change-Id: I8406908c1c0c9ce650824126aa44e96fddf2d20a
Signed-off-by: David Collins <collinsd@codeaurora.org>