MAINTAINERS: Replace Shawn with Frank as i.MX platform maintainer

MAINTAINERS: Replace Shawn with Frank as i.MX platform maintainer

Shawn is no longer interested in maintaining i.MX platform, and would like
to step down.  On the other hand, Frank seems to be the best successor
for this role.

 - He has been one of the most outstanding contributors to i.MX platform
   in the recent years.

 - He has been actively working as a co-maintainer reviewing i.MX
   patches and keep the platform support in good shape.

 - He works for NXP and could be the bridge and coordinator between
   NXP internal developers and community contributors.

Acked-by: Peng Fan <peng.fan@nxp.com>
Acked-by: Daniel Baluta <daniel.baluta@nxp.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Acked-by: Dong Aisheng <aisheng.dong@nxp.com>
Reviewed-by: Frank Li <Frank.Li@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>