cpuidle: Convert Qualcomm SPM driver to a generic CPUidle driver

The Qualcomm SPM cpuidle driver seems to be the last driver still
using the generic ARM CPUidle infrastructure.

Converting it actually allows us to simplify the driver,
and we end up being able to remove more lines than adding new ones:

  - We can parse the CPUidle states in the device tree directly
    with dt_idle_states (and don't need to duplicate that
    functionality into the spm driver).

  - Each "saw" device managed by the SPM driver now directly
    registers its own cpuidle driver, removing the need for
    any global (per cpu) state.

The device tree binding is the same, so the driver stays
compatible with all old device trees.

Signed-off-by: Stephan Gerhold <stephan@gerhold.net>
Reviewed-by: Lina Iyer <ilina@codeaurora.org>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Acked-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>