MAINTAINERS: update Xiubo's email address

My current email address will be gone shortly, update my email to be a
gmail one.

Signed-off-by: Xiubo Li <Li.Xiubo@freescale.com>
Cc: Timur Tabi <timur@tabi.org>
Cc: Takashi Iwai <tiwai@suse.de>
Acked-by: Nicolin Chen <nicoleotsuka@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>