MAINTAINERS: Update Layerscape PCIe driver maintainers list

Change Layerscape PCIe driver maintainers' email address
from freescale to nxp.

Signed-off-by: Minghuan Lian <Minghuan.Lian@nxp.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: Roy Zang <roy.zang@nxp.com>