ARM: Change load tracking scale using sysfs

These functions allow to change the load average period used
in the task load average computation through
/sys/kernel/hmp/load_avg_period_ms. This period is the time
in ms to go from 0 to 0.5 load average while running or the
time from 1 to 0.5 while sleeping.

The default one used is 32 and gives the same load_avg_ratio
computation than without this patch. These functions also allow
to change the up and down threshold of HMP using
/sys/kernel/hmp/{up,down}_threshold. Both must be between 0 and
1024. The thresholds are divided by 1024 before being compared
to the load_avg_ratio.

If /sys/kernel/hmp/load_avg_period_ms is 128 and
/sys/kernel/hmp/up_threshold is 512, a task will be migrated
to a bigger cluster after running for 128ms. Because after
load_avg_period_ms the load average is 0.5 and real up_threshold
us 512 / 1024 = 0.5.

Signed-off-by: Olivier Cozette <olivier.cozette@arm.com>
Signed-off-by: Chris Redpath <chris.redpath@arm.com>

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