MAINTAINERS: rectify entry for XILINX GPIO DRIVER

Commit ba96b2e7 ("dt-bindings: gpio: gpio-xilinx: Convert Xilinx axi
gpio binding to YAML") converts gpio-xilinx.txt to xlnx,gpio-xilinx.yaml,
but missed to adjust its reference in MAINTAINERS.

Hence, ./scripts/get_maintainer.pl --self-test=patterns complains about a
broken reference.

Repair this file reference in XILINX GPIO DRIVER.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>