MAINTAINERS: Add myself as designated reviewer for 8250_dw

It happened several times that some odd fixes either introduced
regressions, or did hack the code instead of addressing a root cause.

Nominate myself to be a designated reviewer for 8250_dw driver.

Cc: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Cc: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>