target/dsp563xx: dsp563xx restore reg support

target/dsp563xx: dsp563xx restore reg support

Added "exist=true" field to the reg_list struct to make access to the
dsp563xx registers again possible. Without it defaults to exist=false
and all the reg related functions will return nothing.

Fixes regression from b5964191f0d2fc3ace607af001df3d57cbfbaf2b

Change-Id: I9c256346735b8d66919c4ba83f528a8afca46ff9
Signed-off-by: Han Hartgers <han.hartgers@gmail.com>
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: http://openocd.zylin.com/5239
Tested-by: jenkins