iomux-v3: Add support for mx6sl LVE bit

iomux-v3: Add support for mx6sl LVE bit

On mx6sl there is a LVE (Low Voltage Enable) bit in the IOMUXC_SW_PAD_CTL
register that can enable or disable low voltage on the pad.

LVE is bit 22 of IOMUXC_SW_PAD_CTL register, but in order to make the
calculation easier we can define it as a flag in bit 1, since this bit is unused.

Add support for it.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Tested-by: Otavio Salvador <otavio@ossystems.com.br>