target: detect correct Cortex-M tar auto increment size

target: detect correct Cortex-M tar auto increment size

The ADIv5 spec guarentees that tar_autoincr_block will be 10bits.
Make this the default for Cortex-M family until we detect a Cortex-M3/M4,
we then change autoincrement to 12bits.

Change-Id: Ie8c89134aa036879bdd8a3c312cee9715dbc6913
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/730
Tested-by: jenkins
Reviewed-by: simon qian <simonqian.openocd@gmail.com>
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>