at91samd: add erase/secure commands, minor fix

Reference code for the SAMD2x disables caching in the NVM controller when
issuing NVM commands.  Let's do this as well to be consistent and safer.

Add a "chip-erase" for the Atmel SAMD targets that issues a complete Chip Erase
via the Device Service Unit (DSU).  This can be used to "unlock" or otherwise
unbrick a chip that can't be halted or inspected, allowing the user to reflash
with new firmware.

Add a "set-security" command which issues an SSB.  Once that's done and the
device is power-cycled, the flash cannot be written to until a "chip-erase" is
issued.  The chip-erase cannot be issued by openocd at this time because
the device will not respond to a request for the DAP IDCODE.

Change-Id: I80122f0bbf7e3aedffe052c1e77d69dc2dba25ed
Signed-off-by: Andrey Yurovsky <yurovsky@gmail.com>
Reviewed-on: http://openocd.zylin.com/2239
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>

doc/openocd.texi

@@ -5023,6 +5023,30 @@ flash bank $_FLASHNAME aduc702x 0 0 0 0 $_TARGETNAME
 @end example
 @end deffn
 
+@anchor{at91samd}
+@deffn {Flash Driver} at91samd
+@cindex at91samd
+
+@deffn Command {at91samd chip-erase}
+Issues a complete Flash erase via the Device Service Unit (DSU). This can be
+used to erase a chip back to its factory state and does not require the
+processor to be halted.
+@end deffn
+
+@deffn Command {at91samd set-security}
+Secures the Flash via the Set Security Bit (SSB) command. This prevents access
+to the Flash and can only be undone by using the chip-erase command which
+erases the Flash contents and turns off the security bit. Warning: at this
+time, openocd will not be able to communicate with a secured chip and it is
+therefore not possible to chip-erase it without using another tool.
+
+@example
+at91samd set-security enable
+@end example
+@end deffn
+
+@end deffn
+
 @anchor{at91sam3}
 @deffn {Flash Driver} at91sam3
 @cindex at91sam3