armv7a: add d-cache virtual address range flush function

armv7a: add d-cache virtual address range flush function

This patch adds a function for cleaning & invalidating a virtual
address range from the architecture caches down to the point of
coherence.

Change-Id: I4061ab023a3797fabc967f3a34498034841d52c6
Signed-off-by: Matthias Welwarsky <matthias@welwarsky.de>
Reviewed-on: http://openocd.zylin.com/3026
Reviewed-by: Paul Fertser <fercerpav@gmail.com>
Tested-by: jenkins