dt-bindings: mtd: update mtd-physmap.yaml reference

dt-bindings: mtd: update mtd-physmap.yaml reference

Changeset a7a9abac4c49 ("dt-bindings: mtd: Convert mtd-physmap to DT schema")
renamed: Documentation/devicetree/bindings/mtd/mtd-physmap.txt
to: Documentation/devicetree/bindings/mtd/mtd-physmap.yaml.

Update its cross-reference accordingly.

Fixes: a7a9abac4c49 ("dt-bindings: mtd: Convert mtd-physmap to DT schema")
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/ab2ff9ee66110c37691b467ec8b4679e9d426416.1626947923.git.mchehab+huawei@kernel.org
Signed-off-by: Rob Herring <robh@kernel.org>