dt-bindings: spi: spi-cadence: Describe power-domains property

ZynqMP Cadence SPI IP core has own power domain that's why describe it as
optional property.

Signed-off-by: Michal Simek <michal.simek@amd.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/987430ee905fd299fe962663d94f848b341c87df.1691047461.git.michal.simek@amd.com
Signed-off-by: Mark Brown <broonie@kernel.org>