MAINTAINERS: Update entry for Freescale SoC drivers

Add myself as the maintainer for drivers/fsl/soc/ and fix the scope for
device tree bindings.

Signed-off-by: Li Yang <leoyang.li@nxp.com>
Acked-by: Scott Wood <oss@buserror.net>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>