MAINTAINERS: update snps,dw-axi-dmac.yaml reference

Changeset 8c70fb7e ("dt-bindings: dma: Add YAML schemas for dw-axi-dmac")
renamed: Documentation/devicetree/bindings/dma/snps,dw-axi-dmac.txt
to: Documentation/devicetree/bindings/dma/snps,dw-axi-dmac.yaml.

Update its cross-reference accordingly.

Fixes: 8c70fb7e ("dt-bindings: dma: Add YAML schemas for dw-axi-dmac")
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>