MAINTAINERS: update IMX kernel git tree

Change Shawn's email address to his employer, and move IMX git tree to
kernel.org.

Cc: Sascha Hauer <kernel@pengutronix.de>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Olof Johansson <olof@lixom.net>