openocd: speed-up

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

debuggers/openocd/src/target/arm_dpm.c

@@ -265,7 +265,7 @@ int arm_dpm_read_current_registers(struct arm_dpm *dpm)
 	arm_set_cpsr(arm, cpsr);
 
 	/* REVISIT we can probably avoid reading R1..R14, saving time... */
-	for (unsigned i = 1; i < 16; i++) {
+	for (unsigned i = 15; i < 16; i++) { // i = 1
 		r = arm_reg_current(arm, i);
 		if (r->valid)
 			continue;