ARM: dts: mdm: Specify the gpio needed for sd_slot_vreg

Since sd_slot_vreg is a GPIO controlled regulator,
add reference of the gpio which controls it.

This is required to enable the SD card module.

Change-Id: Iee94964abaac2f811407791a678ee240f4ae8579
Signed-off-by: Veerabhadrarao Badiganti <vbadigan@codeaurora.org>