cortex_m: Restore fast register reads if no polling is needed

cortex_m: Restore fast register reads if no polling is needed

If the target is in a state where S_REGRDY polling is necessary (slow
clock, low power state...?), OpenOCD will continue to use the slow
path even if the condition is temporary and the target at a later
point would be capable of fast reads again.

Revert to fast reads if a full register dump can be made without need
for polling any of the registers; presumably it will succeed the next
time too.

Change-Id: I557f0d90b7ce6f9d81aa409b6400fc9c83d16008
Signed-off-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/6678
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>