at91sam4: Adding support for the AT91SAM4S4A.

Added the chip definition for the Atmel AT91SAM4S4A. This chip is a 48-pin
package with 256k flash and 64k ram.

Change-Id: I8ada7d5735e31e0ce086f96f5906c7358464245c
Signed-off-by: Thomas Schmid <thomas@rfranging.com>
Reviewed-on: http://openocd.zylin.com/2254
Tested-by: jenkins
Reviewed-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>