pinctrl: imx: Support i.MX7D

Introudce i.MX7 pinctrl driver support.
For now only i.MX7D supported.
There are two iomux controllers in i.MX7D, iomuxc and iomuxc_lpsr.
To iomuxc_lpsr, ZERO_OFFSET_VALID is set, means offset of mux_reg
and conf_reg can begin at 0.

Signed-off-by: Peng Fan <van.freenix@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>