ARM: dts: sa2150p: enable rgmii level shifter on nand vt som

The NAND based sa2150p SOM has a level shifter which is
enabled through GPIO 16. This commit configures GPIO 16
to low state by-default to enable the shifter.

Change-Id: I770e6b49e9dca2e9d7f6076ef772a279a4c2a25e
Signed-off-by: Rishi Gupta <rishgupt@codeaurora.org>