Introduce pinctrl for i.MX6
1. pinctrl-imx.c is for common usage. It's used by i.MX6/7.
2. Add PINCTRL_IMX PINCTRL_IMX6 Kconfig entry.
3. To the pinctrl_ops implementation, only set_state is implemented.
   To i.MX6/7, the pinctrl dts entry is as following:
&iomuxc {
        pinctrl-names = "default";

        pinctrl_csi1: csi1grp {
                fsl,pins = <
                MX6UL_PAD_CSI_MCLK__CSI_MCLK            0x1b088
                MX6UL_PAD_CSI_PIXCLK__CSI_PIXCLK        0x1b088
                MX6UL_PAD_CSI_VSYNC__CSI_VSYNC          0x1b088
                >;
        };

        [.....]
};
  there is no property named function or groups. So pinctrl_generic_set_state
  can not be used here.
5. This driver is a simple implementation for i.mx iomux controller,
   only parse the fsl,pins property and write value to registers.
6. With DEBUG enabled, we can see log when "i2c bus 0":
   "
   set_state_simple op missing
   imx_pinctrl_set_state: i2c1grp
   mux_reg 0x14c, conf_reg 0x3bc, input_reg 0x5d8, mux_mode 0x0, input_val 0x1, config_val 0x4000007f
   write mux: offset 0x14c val 0x10
   select_input: offset 0x5d8 val 0x1
   write config: offset 0x3bc val 0x7f
   mux_reg 0x148, conf_reg 0x3b8, input_reg 0x5d4, mux_mode 0x0, input_val 0x1, config_val 0x4000007f
   write mux: offset 0x148 val 0x10
   select_input: offset 0x5d4 val 0x1
   write config: offset 0x3b8 val 0x7f
   "
   this means imx6 pinctrl driver works as expected.

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