USB: bcma: add support for controlling bus power through GPIO

On some boards a GPIO is needed to activate USB controller. Make it
possible to specify such a GPIO in device tree.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>