arm/dt: msm8974: enable write protect support for sd card

Enable the pmic gpio 29 and set it use 1.8 V as reference since
this write protect gpio works at this voltage level.

Change-Id: Ida2a48d9d0f99acc2a05365a53748c64d620ae84
Signed-off-by: Krishna Konda <kkonda@codeaurora.org>