MAINTAINERS: add all files matching "imx" and "mxs" to the IMX entry

This way also the spi, i2c and other drivers are covered. There are some
x86 specific chips in media/i2c that include imx in their name which
don't have anything to do with the NXP platforms and so are explicitly
excluded.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>