dt-bindings: fix vendor prefix for Abracon

Commit 446810f2 ("of: add vendor prefix for Abracon Corporation")
claimed that "abcn" was used as the vendor prefix while in fact "abracon"
was used in the subsequent commits. It is also the only prefix used in the
tree.

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
[robh: fix alphabetical order]
Signed-off-by: Rob Herring <robh@kernel.org>