dm: gpio: Add driver for MPC85XX GPIO controller

This patch adds a driver for the built-in GPIO controller of the MPC85XX
SoC (probably supporting other PowerQUICC III SoCs as well).

Each GPIO bank is identified by its own entry in the device tree, i.e.

gpio-controller@fc00 {
      #gpio-cells = <2>;
      compatible = "fsl,pq3-gpio";
      reg = <0xfc00 0x100>
}

By default, each bank is assumed to have 32 GPIOs, but the ngpios
setting is honored, so the number of GPIOs for each bank in configurable
to match the actual GPIO count of the SoC (e.g. the 32/32/23 banks of
the P1022 SoC).

The usual functions of GPIO drivers (setting input/output mode and output
value setting) are supported.

The driver has been tested on MPC85XX, but it is likely that other
PowerQUICC III devices will work as well.

Signed-off-by: Mario Six <mario.six@gdsys.cc>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: York Sun <york.sun@nxp.com>