doc: gpio-mvebu: fix broken reference to cp110-system-controller0.txt file

The cp110-system-controller0.txt file was renamed to
cp110-system-controller.txt.

Fixes: 4aa54969 ("dt-bindings: cp110: rename cp110 syscon file")
Signed-off-by: Otto Sabart <ottosabart@seberm.com>
Signed-off-by: Rob Herring <robh@kernel.org>