dt-bindings: add broadcom (formerly plx technology) vendor prefix

PLX Technology meanwhile got bought by Broadcom Corporation but the
vendor prefix plx is still used in 8 current device trees. This silences
the following checkpatch.pl warning:

WARNING: DT compatible string vendor "plx" appears un-documented
 -- check ./Documentation/devicetree/bindings/vendor-prefixes.txt

Signed-off-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Reviewed-by: Andreas Färber <afaerber@suse.de>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>