media: i2c: max9286: Add support for port regulators

Allow users to use one PoC regulator per port, instead of a global
regulator.

The properties '^port[0-3]-poc-supply$' in the DT node are used to
indicate the regulators for individual ports.

Signed-off-by: Thomas Nizan <tnizan@witekio.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Jacopo Mondi <jacopo.mondi@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>