target/arc_cmd: Improve argument checks for commands

target/arc_cmd: Improve argument checks for commands

Add more argument check for "add-reg" command.

Changes since first revision:
-Removed arguments limitation(50 maximum) for "arc_set_reg_exists".

Changes:
25.03:
Removed inconsistency in "add-reg" function. Actually
"-type" option is optional and if it is not set,
register type is "int".

Change-Id: Ia21e6baf4fbda162f7811cd0fe305fc86ddafcfd
Signed-off-by: Evgeniy Didin <didin@synopsys.com>
Reviewed-on: http://openocd.zylin.com/5523
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-by: Oleksij Rempel <linux@rempel-privat.de>