ARM: dts: imx6sl-evk: Keep VGEN1 regulator always enabled

On imx6sl-evk board the VGEN1 regulator powers up the NVCC_1P2V domain of the
imx6sl SoC, so we need to keep it always powered.

According to imx6sl datasheet the GPIO block has three supplies:
NVCC33_IO, NVCC18_IO and NVCC_1P2V and it states that:
"All digital I/O supplies (NVCC_xxxx) must be powered under normal conditions
whether the associated I/O pins are in use or not"

This problem has been observed by the fact that a GPIO connected to an LED could
not work when the PMIC driver was enabled.

Keeping VGEN1 regulator always enabled fixes the problem.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>