Cortex-A8: stop using CP15 ops

Cortex-A8: stop using CP15 ops

There were two chunks of Cortex-A8 code which called the
ARMv7-A CP15 operations; get rid of them, helping prepare
to remove those methods completely:

 - post_debug_entry() can use the mrc() method to read
   its two registers.

 - write_memory() can use dpm->instr_write_data_r0() to
   flush the ICache and DCache ... doing it this way is
   actually faster since it reduces per-write overhead.

Note that the mrc() method parameters are re-ordered with
respect to the ARM instruction documentation, so that part
can be confusing.

Cleaned up the layout and comments in those areas a bit.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>