dt-bindings: display: bcm2835: add optional property power-domains

dt-bindings: display: bcm2835: add optional property power-domains

The Raspberry Pi boards with BCM283x needs control of the power domains
to get display components running. DT schema warns us that it's used, but not
documented as a optional property:

hdmi@7e902000: 'power-domains' does not match any of the regexes: ...

Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/1628334401-6577-4-git-send-email-stefan.wahren@i2se.com
Signed-off-by: Nicolas Saenz Julienne <nsaenz@kernel.org>