Add ft6236 touchscreen driver (FT6x06 and FT6x36)

[notro]

This driver is used with some capacitive touch screens and has been in linux-next for three weeks now.

Example DT overlay fragment:

    fragment@3 {
        target = <&i2c1>;
        __overlay__ {
            status = "okay";

            /* needed to avoid dtc warning */
            #address-cells = <1>;
            #size-cells = <0>;

            ft6236: ft6236@38 {
                compatible = "focaltech,ft6236";
                reg = <0x38>;

                interrupt-parent = <&gpio>;
                interrupts = <24 2>;

                touchscreen-size-x = <320>;
                touchscreen-size-y = <480>;
            };
        };
    };

    __overrides__ {
        invx =      <&ft6236>,"touchscreen-inverted-x?";
        invy =      <&ft6236>,"touchscreen-inverted-y?";
        swapxy =    <&ft6236>,"touchscreen-swapped-x-y?";
    };

[popcornmix]

Okay with me. @pelwell ?