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>

MAINTAINERS

@@ -209,6 +209,7 @@ M:	Lukasz Majewski <l.majewski@samsung.com>
 S:	Maintained
 T:	git git://git.denx.de/u-boot-dfu.git
 F:	drivers/dfu/
+F:	drivers/usb/gadget/
 
 DRIVER MODEL
 M:	Simon Glass <sjg@chromium.org>