MAINTAINERS: add Freescale Layerscape ARM architecture entry

I have been collecting Freescale Layerscape device tree source files for
a while.  Add a MAINTAINERS entry for it, so that relevant patches can
reach my mail box.

Signed-off-by: Shawn Guo <shawnguo@kernel.org>
Acked-by: Li Yang <leoyang.li@nxp.com>