dt-bindings: vendor-prefixes: document Powkiddy

Document Powkiddy (https://powkiddy.com/

).

Signed-off-by: Chris Morgan <macromorgan@hotmail.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20231013183918.225666-2-macroalpha82@gmail.com