dt-bindings: Fix reference in submitting-patches.rst to the DT ABI doc

submitting-patches.rst has a stale reference to ABI.txt. Fix this with a
proper rSt link.

Cc: Frank Rowand <frowand.list@gmail.com>
Cc: Mauro Carvalho Chehab <mchehab@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>
Reviewed-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/20210325164713.1296407-2-robh@kernel.org