MIPS: BCM47XX: Enable USB power on Netgear WNDR3400v3

Setting GPIO 21 high seems to be required to enable power to USB ports
on the WNDR3400v3. As there is already similar code for WNR3500L,
make the existing USB power GPIO code generic and use that.

Signed-off-by: Tuomas Tynkkynen <tuomas.tynkkynen@iki.fi>
Acked-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: Paul Burton <paul.burton@mips.com>
Patchwork: https://patchwork.linux-mips.org/patch/20259/
Cc: Rafał Miłecki <zajec5@gmail.com>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org