kinetis : Add flash sector size detection for K21 MCU.

kinetis : Add flash sector size detection for K21 MCU.

The current way of detecting the sector size of the internal
flash does not work for all Kinetis MCUs. Add support for the
K21 flash by detecting the specific model from the SDID register
and picking the correct sector size based on that.

Change-Id: I41f678353e45a44561af8a3398bdd4479dfeea4f
Signed-off-by: Per Ekman <pekenator@gmail.com>
Reviewed-on: http://openocd.zylin.com/1764
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>