MAINTAINERS: i.MX: Add board/freescale/*mx* path

Pass the board/freescale/*mx*/ path as files maintained by Stefano
Babic.

While this is not ideal and does not cover all the i.MX board cases,
it gives at least a better hint for the /scripts/get_maintainer.pl
tool.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>