dt-bindings: vendor-prefixes: Deprecate "ste" and "st-ericsson"

Until now, device tree bindings for ST-Ericsson have been added
inconsistently with one of 3 possible vendor prefixes.

"stericsson" is the most commonly used vendor prefix,
so deprecate "ste" and "st-ericsson".

Suggested-by: Rob Herring <robh+dt@kernel.org>
Signed-off-by: Stephan Gerhold <stephan@gerhold.net>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Rob Herring <robh@kernel.org>