Document the 'maxim,reverse-channel-microvolt' vendor property in the
bindings document of the max9286 driver.

The newly introduced property allows to specify the initial
configuration of the GMSL reverse control channel to accommodate
remote serializers pre-programmed with the high threshold power
supply noise immunity enabled.
Reviewed-by: NRob Herring <robh@kernel.org>
Reviewed-by: NLaurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: NKieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Signed-off-by: NJacopo Mondi <jacopo+renesas@jmondi.org>
Signed-off-by: NSakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: NMauro Carvalho Chehab <mchehab+huawei@kernel.org>