flash/nor/stellaris: improve support for Tiva C (Blizzard and Snowflake)

This should make current Tiva C parts usable apart from the protection.

Runtime tested on TM4C123GXL (Blizzard) and TM4C1294XL (Snowflake).

Change-Id: Ia64e9d39fbd2b7049578bbfade72435e5203ddf5
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/2257
Tested-by: jenkins
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>