MAINTAINERS: Add F: drivers/usb/gadget to DFU custodian responsibility

After discussion during the last u-boot mini summit with USB maintainer -
Marek Vasut - it has been decided, that gadget development should be
coordinated by DFU custodian.

Such patch formalizes current development status.

Signed-off-by: Lukasz Majewski <l.majewski@samsung.com>