ARM: dts: msm: Add SD card slot external regulator for msmkrypton

The SD card slot power is supplied by an external regulator controlled
by MSM GPIO 51. Add the device tree node for it.

Change-Id: I3efdd1cb3a47b26c0979593b029d925678943184
Signed-off-by: Xiaozhe Shi <xiaozhes@codeaurora.org>