cortex_m: mem_ap access functions take number of bytes, not words

cortex_m: mem_ap access functions take number of bytes, not words

Accessing one byte of memory using a 16-bit access is not well defined.
The current implementation is forgiving and rounds up, but it should not
be relied upon.

Also, I suspect this code might fail if the byte order differs between
target and host, but I have no way of verifying it so I left it as it is.

Change-Id: I8d6a511151a194ed419f141703201f0632d84fc8
Signed-off-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Reviewed-on: http://openocd.zylin.com/1657
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>