Schematic indicates GPIO5_7 is to be used for VTT regulator control
rather than GPIO0_21 so modify enable_vtt_regulator to reflect this.
Without this some boards will experience DDR3 corruption and fail to
boot.

Signed-off-by: Dave Gerlach <d-gerlach@ti.com>
[trini: Rework patch against mainline]
Signed-off-by: Tom Rini <trini@ti.com>