ARM: dts: msm: add proxy vote for DSI supplies for SDM670

For continuous splash screen feature the display needs to
be kept alive during the kernel boot up. Add proxy vote for
all the regulator supplies needed to keep the display alive.

Change-Id: I5823b2fe5dec58233acca9c515166cf07a1f18bd
Signed-off-by: Sandeep Panda <spanda@codeaurora.org>