nrf51: Implement the support for Nordic's nRF51 devices

Add support for Nordic's nRF51 chip series. Tested with nRF51822.

Change-Id: Id70f6fd76888cc595a353aefb84d25c4cd325d7d
Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Reviewed-on: http://openocd.zylin.com/1945
Tested-by: jenkins
Reviewed-by: Andrey Yurovsky <yurovsky@gmail.com>
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>

doc/openocd.texi

@@ -5687,6 +5687,23 @@ unlock str9 device.
 
 @end deffn
 
+@deffn {Flash Driver} nrf51
+All members of the nRF51 microcontroller families from Nordic Semiconductor
+include internal flash and use ARM Cortex-M0 core.
+
+@example
+flash bank $_FLASHNAME nrf51 0 0x00000000 0 0 $_TARGETNAME
+@end example
+
+Some nrf51-specific commands are defined:
+
+@deffn Command {nrf51 mass_erase}
+Erases the contents of the code memory and user information
+configuration registers as well. It must be noted that this command
+works only for chips that do not have factory pre-programmed region 0
+code.
+@end deffn
+@end deffn
 
 @section mFlash