dt-bindings: don't use ../dir for doc references

As documents have been renamed and moved around, their
references will break, but this will be unnoticed, as the
script which checks for it won't handle "../" references.

So, replace them by the full patch.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/68d3a1244119d1f2829c375b0ef554cf348bc89f.1617972339.git.mchehab+huawei@kernel.org
Signed-off-by: Rob Herring <robh@kernel.org>