media: dt-bindings: media: i2c: max9286: Add property to select I2C speed

The I2C speed on the remote side (the I2C master bus of the connected
serializers) is configurable, and doesn't need to match the speed of the
local bus (the slave bus of the MAX9286). All remote buses must use the
same speed, and the MAX9286 needs to be programmed accordingly. Add a
new DT property to select the speed to make it configurable.

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