dt-bindings: mxsfb: Document i.MX8M/i.MX6SX/i.MX6SL power-domains property

The power-domains property is mandatory on i.MX8M Mini, Nano, Plus
and i.MX6SX, i.MX6SL. 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>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Liu Ying <victor.liu@nxp.com>
Link: https://lore.kernel.org/r/20221213132006.6446-1-marex@denx.de


Signed-off-by: Rob Herring <robh@kernel.org>