dt-bindings: wakeup-source: update reference to m8921-keypad.yaml

dt-bindings: wakeup-source: update reference to m8921-keypad.yaml

commit 53ed3233e6b5 ("dt-bindings: input: qcom,pm8921-keypad: convert to
YAML format") resulted in a renaming of the output .txt file from
qcom,pm8xxx-keypad.txt to qcom,pm8921-keypad.yaml.

This patch makes a corresponding update to the link to that .txt file
in wakeup-source.txt.

Flagged by make htmldocs:
Warning: Documentation/devicetree/bindings/power/wakeup-source.txt references a file that doesn't exist: Documentation/devicetree/bindings/input/qcom,pm8xxx-keypad.txt

Signed-off-by: Simon Horman <horms@kernel.org>
Link: https://lore.kernel.org/r/20240908-keypad-wakeup-ref-v1-1-762e4641468a@kernel.org
Signed-off-by: Rob Herring (Arm) <robh@kernel.org>