arm920t: memory writes were broken when MMU was disabled

arm920t: memory writes were broken when MMU was disabled

To support breakpoints, flush data cache line and invalidate
instruction cache when 4 and 2 byte words are written.

The previous code was trying to write directly to the physical
memory, which was buggy and had a number of other situations
that were not handled.

Signed-off-by: Øyvind Harboe <oyvind.harboe@zylin.com>