cpuidle: move to use bus_get_dev_root()

cpuidle: move to use bus_get_dev_root()

Direct access to the struct bus_type dev_root pointer is going away soon
so replace that with a call to bus_get_dev_root() instead, which is what
it is there for.

This allows us to clean up the cpuidle_add_interface() call a bit as it
was only called in one place, with the same argument so just put that
into the function itself.  Note that cpuidle_remove_interface() should
also probably be removed in the future as there are no callers of it for
some reason.

Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
Cc: linux-pm@vger.kernel.org
Acked-by: Rafael J. Wysocki <rafael@kernel.org>
Link: https://lore.kernel.org/r/20230322090557.2943479-1-gregkh@linuxfoundation.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>