Add support for flashing newer members of the NXP Kinetis KW family

Supported devices:
 - KW20Z
 - KW30Z
 - KW40Z
 - KW21Z
 - KW31Z
 - KW41Z

The earlier KW2xD and KW01Z devices are already supported by the code
for the older K-series.

Verified working on the FRDM-KW41Z development board.
Tested flashing both via GDB `load` and directly via OpenOCD flash
write commands.

Change-Id: I73eae477127a8b54a33005b3b526b5439450a808
Signed-off-by: Joakim Nohlgård <joakim.nohlgard@eistec.se>
Reviewed-on: http://openocd.zylin.com/4104


Tested-by: jenkins
Reviewed-by: Johann Fischer <johann_fischer@posteo.de>
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>