MAINTAINERS: Add Lucas Stach as co-maintainer for i.MX6 PCI driver

Also drop Shawn Guo per his request.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Shawn Guo <shawn.guo@freescale.com>
Acked-by: Richard Zhu <r65037@freescale.com>
Acked-by: Jingoo Han <jg1.han@samsung.com>