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>

MAINTAINERS

@@ -6870,7 +6870,7 @@ F:	arch/x86/kernel/quirks.c
 
 PCI DRIVER FOR IMX6
 M:	Richard Zhu <r65037@freescale.com>
-M:	Shawn Guo <shawn.guo@freescale.com>
+M:	Lucas Stach <l.stach@pengutronix.de>
 L:	linux-pci@vger.kernel.org
 L:	linux-arm-kernel@lists.infradead.org (moderated for non-subscribers)
 S:	Maintained