brcm47xx: add function to get a gpio pin by name

bcm47xx_nvram_gpio_pin() returns a gpio pin number for a given name based
on configuration data in nvram.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>


git-svn-id: svn://svn.openwrt.org/openwrt@37644 3c298f89-4303-0410-b956-a3cf2f4a3e73