docs: dt: update writing-schema.rst references

Changeset b83db5b8 ("docs: dt: Group DT docs into relevant sub-sections")
renamed: Documentation/devicetree/writing-schema.rst
to: Documentation/devicetree/bindings/writing-schema.rst.

Update the cross-references accordingly.

Fixes: b83db5b8 ("docs: dt: Group DT docs into relevant sub-sections")
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/7cfddf303f1508d26f90d87546d3812faebfc5ba.1617279356.git.mchehab+huawei@kernel.org


Signed-off-by: Rob Herring <robh@kernel.org>