Normally OpenOCD reads all GP-Registers at halt, which is slow.
Restriction to only necessary Registers 1 and 15.
In the normal use case of OpenOCD this performance reduction does
not matter, because debugging does not need to be optimized for
performance. It just has to be faster than the human perception.
If we need to get the values of the registers 2 to 14, these are
fetched on demand per register.
Change-Id: I8587fbcc41c18722baebca1cec074188fe4cdebb
