MAINTAINERS: Update Richard Zhu's email address

The old cryptic address bounces, fix it by using a properly working one.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Richard Zhu <Richard.Zhu@freescale.com>