imx-common: enlarge mux width to 4

For i.MX6, the mux width is 4, not 3. So enlarge the width.
IOMUX_CONFIG_LPSR is changed from 0x8 to 0x20 to not use bit 3 of mux.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Cc: Stefano Babic <sbabic@denx.de>