mx6: soc: Disable VDDPU regulator

As U-boot does not use GPU/VPU peripherals, shutdown the VDDPU regulator
in order to save power.

Signed-off-by: Anson Huang <b20788@freescale.com>
Signed-off-by: Jason Liu <r64343@freescale.com>
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>