dt-bindings: remoteproc: Fix dead link to Keystone DSP GPIO binding

The old text binding 'gpio-dsp-keystone.txt' was replaced by a DT schema in
commit aff0a1701b020c8e6b172f28828fd4f3e6eed41a
("dt-bindings: gpio: Convert ti,keystone-dsp-gpio to DT schema").

Update the reference to point to the new file.

Signed-off-by: Soham Metha <[e-mail]>
Acked-by: Krzysztof Kozlowski <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Mathieu Poirier <[e-mail]>

Documentation/devicetree/bindings/remoteproc/ti,keystone-rproc.txt

···
 - kick-gpios: 		Should specify the gpio device needed for the virtio IPC
 			stack. This will be used to interrupt the remote processor.
 			The gpio device to be used is as per the bindings in,
-			Documentation/devicetree/bindings/gpio/gpio-dsp-keystone.txt
+			Documentation/devicetree/bindings/gpio/ti,keystone-dsp-gpio.yaml
 
 SoC-specific Required properties:
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