media: dt-bindings: media: imx7-csi: Document i.MX8M power-domains property

The power-domains property is mandatory on i.MX8M Quad and Mini.
Document the property and mark it as required on the aforementioned
variants of the IP, present in those SoCs.

Signed-off-by: Marek Vasut <marex@denx.de>
Acked-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>