dt-bindings: power: Rename back power_domain.txt bindings to fix references

With split of power domain controller bindings to power-domain.yaml,
the consumer part was renamed to power-domain.txt breaking the
references.  Undo the renaming.

Reported-by: Geert Uytterhoeven <geert+renesas@glider.be>
Fixes: 5279a3d8 ("dt-bindings: power: Convert Generic Power Domain bindings to json-schema")
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Rob Herring <robh@kernel.org>