gpio: dt-bindings: add ibm,ppc4xx-gpio binding

This patch adds binding information for IBM/AMCC/APM GPIO
Controllers of the PowerPC 4XX series and compatible SoCs.

The "PowerPC 405EP Embedded Processor Data Sheet" has the
following to say about the GPIO controllers: "

 - Controller functions and GPIO registers are programmed
   and accessed via memory-mapped OPB bus master accesses

 - All GPIOs are pin-shared with other functions. DCRs control
   whether a particular pin that has GPIO capabilities acts
   as a GPIO or is used for another purpose.

 - Each GPIO outputs is separately programmable to emulate
   an open-drain driver (i.e. drives to zero, threestated if
   output bit is 1)

"
The ppc4xx_gpio.c driver is part of the platform/sysdev drivers
in arch/powerpc/sysdev.

Signed-off-by: Christian Lamparter <chunkeey@googlemail.com>
Signed-off-by: Rob Herring <robh@kernel.org>

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