MAINTAINERS: Add entry for bcm63xx/bcm33xx UDC gadget driver

MAINTAINERS: Add entry for bcm63xx/bcm33xx UDC gadget driver

This hardware shows up on the newly-supported BCM3384 cable chip, as well
as several old BCM63xx DSL chips.

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
Cc: f.fainelli@gmail.com
Cc: mbizon@freebox.fr
Cc: jogo@openwrt.org
Cc: jfraser@broadcom.com
Cc: linux-mips@linux-mips.org
Cc: devicetree@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/8172/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>