MAINTAINERS: update entry for atmel_serial driver

Change maintainer for the serial driver found on most of the
Microchip / Atmel MPUs and take advantage of the move to rename
and reorder the entry.
I'm happy that Richard is taking over the maintenance of this
driver.

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Acked-by: Richard Genoud <richard.genoud@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>