Add support for the ATMEL SAM G55 Xplained Pro board and CPU.

Change-Id: Iffe59dcf9f2cb1f5949c37d11fe0d2141a47f8da
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: http://openocd.zylin.com/3922


Tested-by: jenkins
Reviewed-by: Leo Zhang <liang.zhang@microchip.com>
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>