dt-bindings: arm: update vexpress-config.yaml references

Changeset 7e8339b5 ("dt-bindings: arm: convert vexpress-config to DT schema")
renamed: Documentation/devicetree/bindings/arm/vexpress-sysreg.txt
to: Documentation/devicetree/bindings/arm/vexpress-config.yaml.

Update the cross-references accordingly.

Fixes: 7e8339b5

 ("dt-bindings: arm: convert vexpress-config to DT schema")
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>
Acked-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/7020edd9e183652249fc95bf61a1055cc342a4dc.1654529011.git.mchehab@kernel.org