at91samd: handle reset run/halt in DSU

Atmel introduced a "Device Service Unit" (DSU) that holds the CPU
in reset if TCK is low when srst (RESET_N) is deasserted.
Function is similar to SMAP in ATSAM4L, see http://openocd.zylin.com/2604

Atmel's EDBG adapter handles DSU reset correctly without this change.

An ordinary SWD adapter leaves TCK in its default state, low.
So without this change any use of sysresetreq or srst
locks the chip in reset state until power is cycled.

A new function dsu_reset_deassert is called as reset-deassert-post event handler.
It optionally prepares reset vector catch and DSU reset is released then.

Additionally SWD clock comment is fixed in at91samdXX.cfg and clock is
lowered a bit to ensure a margin for RC oscillator frequency deviation.
adapter_nsrst_delay 100 is commented out because is no more necessary after
http://openocd.zylin.com/2601

Change-Id: I42e99b1b245f766616c0a0d939f60612c29bd16c
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: http://openocd.zylin.com/2778
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>

doc/openocd.texi

@@ -4933,6 +4933,9 @@ flash bank $_FLASHNAME aduc702x 0 0 0 0 $_TARGETNAME
 @anchor{at91samd}
 @deffn {Flash Driver} at91samd
 @cindex at91samd
+All members of the ATSAMD, ATSAMR, ATSAML and ATSAMC microcontroller
+families from Atmel include internal flash and use ARM's Cortex-M0+ core.
+This driver uses the same cmd names/syntax as @xref{at91sam3}.
 
 @deffn Command {at91samd chip-erase}
 Issues a complete Flash erase via the Device Service Unit (DSU). This can be
@@ -4980,6 +4983,12 @@ at91samd bootloader 16384
 @end example
 @end deffn
 
+@deffn Command {at91samd dsu_reset_deassert}
+This command releases internal reset held by DSU
+and prepares reset vector catch in case of reset halt.
+Command is used internally in event event reset-deassert-post.
+@end deffn
+
 @end deffn
 
 @anchor{at91sam3}