MIPS: RB532: GPIO register offsets are relative to GPIOBASE

This patch fixes the wrong use of GPIO register offsets
in devices.c. To avoid further problems, use gpio_get_value
to return the NAND status instead of our own expanded code.

Also define the zero offset of the alternate function register to allow
consistent access.

Signed-off-by: Florian Fainelli <florian@openwrt.org>
Signed-off-by: Phil Sutter <n0-1@freewrt.org>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>