MAINTAINERS: Update mx6 PCI driver maintainer's email

Use Shawn's email address from Freescale.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Shawn Guo <shawn.guo@freescale.com>