dt-bindings: vendor-prefixes: Add abt vendor prefix

Add prefix for ShenZhen Asia Better Technology Ltd.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20201101093150.8071-2-paul@crapouillou.net