intel_idle: Use subsys_initcall_sync() for initialization

It is not necessary to wait until the device_initcall() stage with
intel_idle initialization.  All of its dependencies are met after
all subsys_initcall()s have run, so subsys_initcall_sync() can be
used for initializing it.

It is also better to ensure that intel_idle will always initialize
before the ACPI processor driver that uses module_init() for its
initialization.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Tested-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Link: https://patch.msgid.link/2994397.e9J7NaK4W3@rjwysocki.net