ARM: DRA: Enable VTT regulator

DRA7 evm REV G and later boards uses a vtt regulator for DDR3 termination
and this is controlled by gpio7_11. Configuring gpio7_11.
The pad A22(offset 0x3b4) is used by gpio7_11 on REV G and later boards,
and left unused on previous boards, so it is safe enough to enable gpio
on all DRA7 boards.

Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>