media: dt-bindings: mt9p031: Add MT9P006 compatible string

Add compatible for MT9P006 sensor, which is older variant of MT9P031
and compatible with this driver.

Signed-off-by: Marek Vasut <marex@denx.de>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>