Re-jiggle the GPIO code a little to remove the hard-coded AR71xx GPIO config and function mask setup.
Re-jiggle the GPIO code a little to remove the hard-coded AR71xx GPIO
config and function mask setup.

* "gpiomask" now specifies which GPIO pins to enable, for devices to bind to.
* "function_set" allows bits in the function register to be set at GPIO setup.
* "function_clear" allows bits in the function register to be cleared at
  GPIO setup.

The function_set/function_clear bits allow for individual GPIO pins to either
drive a GPIO line or an alternate function - eg USB, JTAG, etc. This allows
for things like CS1/CS2 be enabled for those boards w/ >1 SPI device connected,
or disabling JTAG for the AR7240 (which is apparently needed ..)

I've verified this on the AR71xx.

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