MAINTAINERS: Use a proper mailinglist for NXP i.MX development

So far we used an internal linux-imx@nxp.com email address to
gather all patches related to NXP i.MX development.

Let's switch to an open mailing list that provides ability
for people from the community to subscribe and also have
a proper archive.

List interface at: https://lists.linux.dev.
Archive is at: https://lore.kernel.org/imx/

Signed-off-by: Daniel Baluta <daniel.baluta@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>