[POWERPC] Add Timur Tabi to the MAINTAINERS file

Add Timur Tabi as the maintainer for the Freescale QE library, the
Freescale QE UART device driver, the Freescale SOC sound drivers, and
the Crystal Semiconductor CS4270 device driver.

Signed-off-by: Timur Tabi <timur@freescale.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>