MIPS: RB532: provide GPIO_BUILTIN_NR and irq_to_gpio/gpio_to_irq

This patchs defines the number of built-in the GPIOs present
on the SoC as Documentation/gpio.txt recommends to do.
Define irq_to_gpio/gpio_to_irq to return the right values so that
it fixes a compilation error on drivers/gpio/gpiolib.c
when enabling debugfs.

Signed-off-by: Florian Fainelli <florian@openwrt.org>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>