pinctrl: imx: Fix select input issue

The pinctrl supports to set any bit in input register on iMX6 if
the MSB of input value is 0xff. But the driver uses signed int for
input value, so when executing the codes below, it won't meet.
Because this is arithmetic right shift.

    if (input_val >> 24 == 0xff)

Fix the issue by changing the input_val, config_val and mux_mode to u32.

Signed-off-by: Ye Li <ye.li@nxp.com>
Reviewed-by: Fugang Duan <fugang.duan@nxp.com>
Reviewed-by: Peng Fan <peng.fan@nxp.com>