[ARM] 5373/2: Add gpiolib support to AT91

Add support for gpiolib, including debugfs output, to the AT91 family.
The at91_get/set_gpio_value calls still exist since they are used by the
atmel serial driver.

Signed-off-by: Ryan Mallon <ryan@bluewatersys.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>