MIPS: BCM47XX: Get GPIO pin from nvram configuration

The nvram contains some gpio configuration for boards. It is stored in
a gpio<number>=name format e.g.
gpio8=wps_button
gpio4=robo_reset

This patches adds a function to parse these entries, so other driver
can use it.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/5841/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>