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.