MAINTAINERS: update ALLWINNER CPUFREQ DRIVER entry

Commit b30d8cf5

 ("dt-bindings: opp: Convert Allwinner H6 OPP to a
schema") converted in Documentation/devicetree/bindings/opp/ the file
sun50i-nvmem-cpufreq.txt to allwinner,sun50i-h6-operating-points.yaml.

Since then, ./scripts/get_maintainer.pl --self-test complains:

  warning: no file matches \
  F: Documentation/devicetree/bindings/opp/sun50i-nvmem-cpufreq.txt

Adjust the file pattern in the ALLWINNER CPUFREQ DRIVER entry.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Signed-off-by: Rob Herring <robh@kernel.org>