Most things valid for the i.MX8MQ are needed for the i.MX8MM as well, so
add a common macro which returns true for both SoCs and use it where
appropriate. Also we have to search for the i.MX header in a bigger area
as it is at offset 33KiB.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>