MAINTAINERS: Declare tools/zynqmp* as Xilinx maintained

The zynqmpimage.c and the new zynqmpbif.c files are all maintained by
Xilinx for the Zynq platforms. Let's match them accordingly
in the MAINTAINERS file.

Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>