targets: fix target_type name for Cortex-A targets

Commit d9ba56c295 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>

src/target/cortex_a.c

@@ -2756,7 +2756,8 @@ static const struct command_registration cortex_a8_command_handlers[] = {
 };
 
 struct target_type cortexa8_target = {
-	.name = "cortex_a8",
+	.name = "cortex_a",
+	.deprecated_name = "cortex_a8",
 
 	.poll = cortex_a8_poll,
 	.arch_state = armv7a_arch_state,