ARM: dts: msm: Update threshold frequencies for MX voting on sdm670

Cast an explicit vote on the MX rail depending on the current
L3 and silver clock frequencies. This is needed to resolve issues
root caused to under-volting memories supplied by this rail.

Change-Id: I7bd27606af0d93de2d5690b1f213b3d6891e279f
Signed-off-by: Odelu Kukatla <okukatla@codeaurora.org>