flash: add stellaris async flash loader

flash: add stellaris async flash loader

This enable the Stellaris flash driver to use the asynchronous
algorithm support.

Speed increase is as follows:
before - wrote 65536 bytes from file test.bin in 5.486040s (11.666 KiB/s)
after - wrote 65536 bytes from file test.bin in 2.274001s (28.144 KiB/s)

Change-Id: I9004c9aadffa1ae3b0cbf908e6549b5b1f794508
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/403
Tested-by: jenkins
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>