dt-bindings: media: imx258: Rename to include vendor prefix

imx258.yaml doesn't include the vendor prefix of sony, so
rename to add it.
Update the id entry and MAINTAINERS to match.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Luis Garcia <git@luigi311.com>
Reviewed-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>