PM / platform_bus: Allow runtime PM by default

Currently the default runtime PM callbacks for platform devices return
-ENOSYS, preventing the use of runtime PM platforms until they have
provided at least a default implementation. This hinders the use of
runtime PM by devices which work with many platforms such as memory
mapped devices, MFDs and on chip IPs shared by multiple architectures.

Change the default implementation to the standard pm_generic_runtime
one, allowing drivers to use runtime PM without per-architecture
changes.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Greg Kroah-Hartman <gregkh@suse.de>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>