MIPS: RB532: Move dev3 init code to devices.c

This code doesn't belong to gpio.c, as it's completely unrelated to
GPIO. As dev1 and dev2 init code is in devices.c, it seems to be a more
adequate place.

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