target/arm_adi_v5: add command "dpreg"

For very low level debug or development around DAP, it is useful
to have direct access to DP registers.

Add command "dpreg" by mimic the syntax of the existing "apreg"
command:
	$dap_name dpreg reg [value]

Change-Id: Ic4ab451eb5e74453133adee61050b4c6f656ffa3
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/4612
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>