arm920t: fix write memory operations with caches enabled

Commit ff5ec942 made this target
always use generic arm7_9 memory write routines for software
breakpoints which resulted in inability to debug and single-step
sources in Gdb when icache is active as generic routine doesn't
invalidate it. This should fix it (and is real-life tested against
Samsung S3C2442). I expect other arm7-9 targets to be affected as
well.

Change-Id: Id7980e370ae4db47ac6b1490321d81ffe85711c0
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/1817


Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>