MLK-11252-01 ARM: imx: dts: add gpio regulator to control the dc-dc voltage on imx6ul evk board

For the i.MX6UL EVK board, according to the latest schematic, use a GPIO pin along with
the 'PMIC_STBY_REQ' pin to control the DC-DC voltage output. the possible voltage output
as below:

PMIC_STBY_REQ = 0, GPIO_DVFS = 0, output is 1.4V (1.375V + 25mV) <Default>
PMIC_STBY_REQ = 0, GPIO_DVFS = 1, output is 1.3V (1.275V + 25mV)
PMIC_STBY_REQ = 1, GPIO_DVFS = 0, output is 0.925V (0.9V + 25mV)

In normal run mode, the voltage output should be changed using the 'GPIO_DVFS' pin,
so add gpio regulator to control it.

Signed-off-by: Bai Ping <b51503@freescale.com>