cortex_m: print 'Cortex-M' rather than 'Cortex-M3'

This file is used by all the Cortex-M family not just Cortex-M3.

Change-Id: Ie8680535b220c66bb8fcd862510407a46a73e8a0
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/1429
Tested-by: jenkins
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>