media: nxp: imx8-isi: Drop partial support for i.MX8QM and i.MX8QXP

Support for the i.MX8QM and i.MX8QXP is only partly implemented and has
never been tested. Furthermore, the corresponding compatible string it
not documented. Drop it, until someone with interest in those platforms
can test and implement it properly.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>