drivers/base/arch_topology: Add infrastructure to store and update instantaneous thermal pressure

Add architecture specific APIs to update and track thermal pressure on a
per CPU basis. A per CPU variable thermal_pressure is introduced to keep
track of instantaneous per CPU thermal pressure. Thermal pressure is the
delta between maximum capacity and capped capacity due to a thermal event.

topology_get_thermal_pressure can be hooked into the scheduler specified
arch_scale_thermal_pressure to retrieve instantaneous thermal pressure of
a CPU.

arch_set_thermal_pressure can be used to update the thermal pressure.

Considering topology_get_thermal_pressure reads thermal_pressure and
arch_set_thermal_pressure writes into thermal_pressure, one can argue for
some sort of locking mechanism to avoid a stale value.  But considering
topology_get_thermal_pressure can be called from a system critical path
like scheduler tick function, a locking mechanism is not ideal. This means
that it is possible the thermal_pressure value used to calculate average
thermal pressure for a CPU can be stale for up to 1 tick period.

Signed-off-by: Thara Gopinath <thara.gopinath@linaro.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lkml.kernel.org/r/20200222005213.3873-4-thara.gopinath@linaro.org

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