workqueue: Debug file to toggle system_highpri_wq prio

To let the RqInvariance:test_load_correctness load-tracking test pass
there has to be a possibility to lower the task priority of the high
prio workers 'kworker/X:YH-/+xx' from 100 (nice=-20) to 120 (nice=0).

Otherwise the cfs_rq load_avg contribution from these tasks is too high
since our test assumes normal task weight, i.e. 1024 (nice=0). Our tests
can only deal with normal task weight.

The /sys/kernel/debug/workqueue/high_prio_wq accepts 0 (to set the prio
of these tasks to 100) and 1 (to set it back to the default value 120).

The issue was introduced by commit ba70aca0b167 ("random: defer fast
pool mixing to worker"). add_interrupt_randomness() now queues
system_highpri_wq workqueue items and these are running in
'kworker/X:YH-/+xx' tasks.

It was first visible on Arm32 TC2 with its timer interrupt but can be
seen on other platforms/interrupts too.

This patch is solely there to let our load-tracing tests pass and not
meant for upstreaming!

Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>