MAINTAINERS: ARM/FREESCALE: merge Layerscape entry into i.MX entry

MAINTAINERS: ARM/FREESCALE: merge Layerscape entry into i.MX entry

Both platforms are maintained by the same people, and the two entries
contain largely identical information. Maintaining separate entries adds
unnecessary duplication.

Additionally, the standalone Layerscape entry does not include the
imx@lists.linux.dev mailing list, causing Layerscape patches to miss
NXP's Patchwork instance. Consolidating the entries ensures patches for
both platforms are routed consistently and reduces future maintenance
overhead.

Signed-off-by: Frank Li <Frank.Li@nxp.com>