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

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 <sohammetha01@gmail.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@oss.qualcomm.com>
Link: https://lore.kernel.org/r/20251203180337.50831-1-sohammetha01@gmail.com
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>