dt-bindings: i2c: cadence: Describe power-domains property

ZynqMP Cadence I2c 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/8774dba53cae5508f9f7aa173fbaf814d97898b1.1691047405.git.michal.simek@amd.com


Signed-off-by: Andi Shyti <andi.shyti@kernel.org>