MIPS: BCM63XX: let board specify an external GPIO to reset PHY

MIPS: BCM63XX: let board specify an external GPIO to reset PHY

Some boards may need to reset their external PHY or switch they are
attached to, add a hook for doing this along with providing custom
linux/gpio.h flags for doing this.

Signed-off-by: Florian Fainelli <florian@openwrt.org>
Cc: linux-mips@linux-mips.org
Cc: cernekee@gmail.com
Cc: jogo@openwrt.org
Cc: Florian Fainelli <florian@openwrt.org>
Patchwork: https://patchwork.linux-mips.org/patch/5501/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>