MAINTAINERS: Update Richard Zhu's email address

FSL emails may become invalid soon, so switch to the NXP one.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Richard Zhu <hongxing.zhu@nxp.com>