sim3x: new flash driver for Silabs SiM3 microcontroller family

This is a new driver for Silicon Laboratories SiM3 microcontroller
family, based on the work of Ladislav Bábel. The driver will try to
detect the type of MCU from the device id register, and if this
fails it will use the flash size from the flash bank command.
Driver added to the documentation and to the README.
TCL script added.

Tests:
* Hardware: SiM3C166 (pre-production) and SiM3U167
* Binary: 4kb, 197kb, 256kb
* Flash protect not tested

Change-Id: I701e0cf505ca8ad99be7f83543fe5055b2f65dcc
Signed-off-by: Andreas Bomholtz <andreas@seluxit.com>
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/2078
Tested-by: jenkins