Add support for the Atmel SAMG53

Add support for the Atmel SAMG53

flash: at91sam4: add support for the SAMG53 family (this also covers the
SAMG51).  The SAMG5x parts have an EEFC (enhanced embedded flash controller)
which seems to be identical to the EFC that the sam4 driver supports.

Add a script for the Xplained Pro G53 board, this has the onboard CMSIS-DAP
debugger and a SAMG53N19.  Tested on this board and chip combination.

Change-Id: I12af50402cd2069b3c7380d92e6fe54816d6c045
Signed-off-by: Andrey Yurovsky <yurovsky@gmail.com>
Reviewed-on: http://openocd.zylin.com/1974
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>