ARM: dts: msm: change vdd as L6 for QRD8937 sku1 DVT

Vdd is changed from L5 to L6 for saving power because of L5
is not able to be voted as disable in suspend which will
cause power leakage.

Change-Id: I30aeee6baa4f2bec81e2f5730a4dd350c3f75733
Signed-off-by: Mao Li <maol@codeaurora.org>