target: fix broken Cortex-R4 support

This regression was caused due to the recent addition of R4 support and
the removal of the bulk_write_memory handler.

Change-Id: Ide692737f235c0e9906becb6f3502ba52c5907aa
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/1246
Tested-by: jenkins