Document power-domains property for adding the Power domain provider.

Link: https://lore.kernel.org/r/20210802091822.16407-1-a-govindraju@ti.comSigned-off-by: Aswath Govindraju <a-govindraju@ti.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>