msm: pil-q6v5-mss: Proxy-vote for vdd_mx when booting the Modem QDSP6

The modem QDSP6 has a hardware requirement that vdd_mx to be >= vdd_mss,
which is set to 1.05V. Currently, this is enforced implicitly as part
of the vdd_mss request sent to the RPM.

Because support for this implicit vote will soon be removed in the RPM,
add an explicit vote for vdd_mx from Linux PIL.

Change-Id: Id8884913b1220b2227e787b994fb9305c2b411ed
Signed-off-by: Matt Wagantall <mattw@codeaurora.org>