Add documentation section for ARCv2

Add documentation section for ARCv2

Changes since v1:
-Moved from http://openocd.zylin.com/#/c/5332/4
 into separate commit.

28.02.2020:
-Removed multiple cpu configuration section, currently
 only ARC EM is supported.

17.03.2020:
-Some cleanup
-For "arc set-reg-exists" command limitize the number of
 arguments (50 maximum).

17.03.2020(v2):
-Revert limitation for "arc set-reg-exist" command

Change-Id: I4b06f89df95f2773bfde6e1bd2ae2b6b880bfaa8
Signed-off-by: Evgeniy Didin <didin@synopsys.com>
Cc: Alexey Brodkin <abrodkin@synopsys.com>
Reviewed-on: http://openocd.zylin.com/5351
Tested-by: jenkins
Reviewed-by: Oleksij Rempel <linux@rempel-privat.de>