MAINTAINERS/serial: Change maintainer of jsm driver

Gabriel will no longer maintain this driver, so I'm adding
myself as maintainer.

Thanks for all your work on jsm driver Gabriel.

Signed-off-by: Guilherme G. Piccoli <gpiccoli@linux.vnet.ibm.com>
Acked-by: Gabriel Krisman Bertazi <gabriel@krisman.be>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>