dt-bindings: phy: update phy-cadence-sierra.yaml reference

Changeset ba2bf1f0

 ("dt-bindings: phy: Add Cadence Sierra PHY bindings in YAML format")
renamed: Documentation/devicetree/bindings/phy/phy-cadence-sierra.txt
to: Documentation/devicetree/bindings/phy/phy-cadence-sierra.yaml.

Update its cross-reference accordingly.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/3550b08d4e8312e7d4a247a3515a93a5f0fd04c5.1610535350.git.mchehab+huawei@kernel.org
Signed-off-by: Vinod Koul <vkoul@kernel.org>