dt-bindings: reset: update st,stih407-powerdown.yaml references

Changeset 2ca065dc ("dt-bindings: reset: st,sti-powerdown: Convert to yaml")
renamed: Documentation/devicetree/bindings/reset/st,sti-powerdown.txt
to: Documentation/devicetree/bindings/reset/st,stih407-powerdown.yaml.

Update the cross-references accordingly.

Fixes: 2ca065dc

 ("dt-bindings: reset: st,sti-powerdown: Convert to yaml")
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/debdd5a9a1bfa0cf1c7e9c45da32edbc2ac2d10a.1654529011.git.mchehab@kernel.org