thermal: power allocator: refactor sustainable power estimation

The sustainable power value might come from the Device Tree or can be
estimated in run time. The sustainable power might be updated by the user
via sysfs interface, which should trigger new estimation of PID
coefficients. There is no need to estimate it every time when the
governor is called and temperature is high. Instead, store the estimated
value and make it available via standard sysfs interface, so it can be
checked from the user-space.

Signed-off-by: Lukasz Luba <lukasz.luba@arm.com>
Reviewed-by: Ionela Voinescu <ionela.voinescu@arm.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20201124161025.27694-3-lukasz.luba@arm.com