dt-bindings: power: Add MediaTek MT6858 power domain controller

Add a new compatible and document bindings for the power domain
controller of the MT6858 SoC.

Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@oss.qualcomm.com>
Signed-off-by: Nikolai Burov <nikolai.burov@jolla.com>
Signed-off-by: Ulf Hansson <ulfh@kernel.org>