cpuidle: mvebu: Update cpuidle thresholds for Armada XP SOCs

cpuidle: mvebu: Update cpuidle thresholds for Armada XP SOCs

Originally, the thresholds used in the cpuidle driver for Armada SOCs
were temporarily chosen, leaving room for improvements.

This commit updates the thresholds for the Armada XP SOCs with values
that positively impact performances:

                                without patch  with patch   vendor kernel
 - iperf localhost (gbit/sec)   ~3.7           ~6.4         ~5.4
 - ioping tmpfs (iops)          ~163k          ~206k        ~179k
 - ioping tmpfs (mib/s)         ~636           ~805         ~699

The idle power consumption is negatively impacted (proportionally less
than the performance gain), and we are still performing better than
the vendor kernel here:

                                without patch   with patch  vendor kernel
 - power consumption idle (W)   ~2.4            ~3.2        ~4.4
 - power consumption busy (W)   ~8.6            ~8.3        ~8.6

There is still room for improvement regarding the value of these
thresholds, they were chosen to mimic the vendor kernel.

This patch only impacts Armada XP SOCs and was tested on Online Labs
C1 boards. A similar approach can be taken to improve the performances
of the Armada 370 and Armada 38x SOCs.

Thanks a lot to Thomas Petazzoni, Gregory Clement and Willy Tarreau
for the discussions and tips around this topic.

Signed-off-by: Sebastien Rannou <mxs@sbrk.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Acked-by: Gregory CLEMENT <gregory.clement@free-electrons.com>