mips: bcm47xx: Enable USB power on Netgear WNDR3400v2

Eric has reported on OpenWrt's bug tracking system[1], that he's not
able to use USB devices on his WNDR3400v2 device after the boot, until
he turns on GPIO #21 manually through sysfs.

1. https://bugs.openwrt.org/index.php?do=details&task_id=2170

Cc: Rafał Miłecki <zajec5@gmail.com>
Cc: Hauke Mehrtens <hauke@hauke-m.de>
Reported-by: Eric Bohlman <ericbohlman@gmail.com>
Tested-by: Eric Bohlman <ericbohlman@gmail.com>
Signed-off-by: Petr Štetiar <ynezz@true.cz>
Signed-off-by: Paul Burton <paul.burton@mips.com>