MIPS: RB532: Export rb532_gpio_set_func()

This kernel symbol provides a way for drivers to switch on alternate
function for a certain GPIO pin. Turning it off is done implicitly when
changing the GPIO direction, as that would be fixed when using the given
pin als alternate function.

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