MLK-14938-7 imx8: Add iomux driver

MLK-14938-7 imx8: Add iomux driver

Wrap the iomux settings through SCU APIs. Provide interfaces to follow
other i.MX platforms.

Users need to define the iomux_cfg_t which combines of three parts:
<PAD id> | <MUX ALT> | <PAD & MUX CONFIG>

<PAD id>   can directly use the values in imx8qm_pins.h
<MUX ALT>  is optional if the select to use default mux. Otherwise using
           MUX_MODE_ALT(x) to set it.
<PAD & MUX CONFIG> is the value for CONFIG, LP_CONFIG, PULL control, Drive Strength in
           Padring control register.

Signed-off-by: Ye Li <ye.li@nxp.com>