docs: dt: fix references to ap806-system-controller.txt

ap806-system-controller.txt was renamed to ap80x-system-controller.txt.

Update its references accordingly.

Fixes: 2537831b ("dt-bindings: ap80x: replace AP806 with AP80x")
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Acked-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Reviewed-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Rob Herring <robh@kernel.org>