targets: fix target_type name for Cortex-A targets

Commit d9ba56c295f057e716519a798bf9cdb4898c24f4 did a bunch of
renaming of cortex_a8 to cortex_a, including the names in config
files. However that introduced a regression as the name in target_type
struct remained unchanged.

This adds the last missing bit: actual renaming of the target name as
understood by OpenOCD.

Also change the (hopefully) last instance of using it in the supplied
config files, namely from imx6.cfg.

Change-Id: Ib9289fc6d946630133ec6e36c20015ccb50acf61
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/1420
Tested-by: jenkins
Reviewed-by: Chris Johns <chrisj@rtems.org>
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>