MAINTAINERS: add dfu.h and dfu.rst to DFU
include/dfu.h and doc/api/dfu.rst belong to the device firmware update
sub-system. So let's add them to DFU in MAINTAINERS.
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Acked-by: Lukasz Majewski <lukma@denx.de>