Per ovti,ov5640.yaml, the OV5640 power supplies are mandatory
properties.

Describe them to fix dt-schema warnings.

As there are two different PMICs used on the imx8mn-evk variants,
describe the DOVDD OV5640 power supply in each board devicetree.
Signed-off-by: Fabio Estevam <festevam@denx.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>