pinctrl: mxs: Add support for i.MX2[38] mxs pinctrl driver

pinctrl: mxs: Add support for i.MX2[38] mxs pinctrl driver

The code responsible for setting proper values in the MUX registers
(in the mxs_pinctrl_set_state()) has been ported from Linux kernel
- SHA1: 17bb763e7eaf tag v5.1.11 from linux-stable.

As the pinctrl node in the imx28.dtsi file has gpio pins nodes as subnodes,
it was necessary to use 'dm_scan_fdt_dev()' (as a .bind method) to also
make them 'visible' by the DM's "gpio_mxs" driver.

Signed-off-by: Lukasz Majewski <lukma@denx.de>
Reviewed-by: Marek Vasut <marex@denx.de>