PM: AVS: Drop the avs directory and the corresponding Kconfig

PM: AVS: Drop the avs directory and the corresponding Kconfig

All avs drivers have now been moved to their corresponding soc specific
directories. Additionally, they don't depend on the POWER_AVS Kconfig
anymore. Therefore, let's simply drop the drivers/power/avs directory
altogether.

Cc: Nishanth Menon <nm@ti.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Reviewed-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>