coresight: docs: Fix a broken reference

coresight: docs: Fix a broken reference

Since the commit in Fixes: tag, "coresight-cpu-debug.txt" has been turned
into "arm,coresight-cpu-debug.yaml".

Update the doc accordingly to avoid a 'make htmldocs' warning

Fixes: 66d052047ca8 ("dt-bindings: arm: Convert CoreSight CPU debug to DT schema")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: James Clark <james.clark@arm.com>
Link: https://lore.kernel.org/r/c7f864854e9e03916017712017ff59132c51c338.1659251193.git.christophe.jaillet@wanadoo.fr
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>