MAINTAINERS: assign systemace driver to Xilinx

Assign systemace driver to Xilinx Zynq to ensure if there is a change that
someone can even test it.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>