MAINTAINERS: Record documentation for Xilinx platforms

Add fragment to cover documenation for Xilinx platforms.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>

MAINTAINERS

@@ -546,6 +546,7 @@ M:	Michal Simek <monstr@monstr.eu>
 S:	Maintained
 T:	git https://gitlab.denx.de/u-boot/custodians/u-boot-microblaze.git
 F:	arch/arm/mach-zynq/
+F:	doc/board/xilinx/
 F:	drivers/clk/clk_zynq.c
 F:	drivers/fpga/zynqpl.c
 F:	drivers/gpio/zynq_gpio.c