diag: Add AXI prefetch related padding for CI controller

The patch adds padding of 256 bytes as a workaround to
get rid of AXI prefetch related h/w issue while writing
data to USB. The padding is applicable only to chipidea
controller.

Change-Id: I51b56ec07ec8815e942324fa1bd7f167b6335b64
Signed-off-by: Mohit Aggarwal <maggarwa@codeaurora.org>