dt-bindings: mfd: rk808: update rockchip,rk808.yaml reference

Changeset 6c38ca03 ("dt-bindings: mfd: rk808: Convert bindings to yaml")
renamed: Documentation/devicetree/bindings/mfd/rk808.txt
to: Documentation/devicetree/bindings/mfd/rockchip,rk808.yaml.

Update its cross-reference accordingly.

Fixes: 6c38ca03

 ("dt-bindings: mfd: rk808: Convert bindings to yaml")
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/417281c270e098eefed763859480014bec75c883.1654529011.git.mchehab@kernel.org