nrf51: Add UICR writing support

SoftDevice stack ihex binary, provided by Nordic expects being able to
write data necessary for its correct operation at the adresses inside UICR.
This patch exposes UICR region of flash as a second bank on the MCU to
facilitate that.

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Change-Id: Idbc140b8de027f60655f78043877b7c054eb06f9
Reviewed-on: http://openocd.zylin.com/2013


Tested-by: jenkins
Reviewed-by: Paul Fertser <fercerpav@gmail.com>