PM: Run-time PM platform device bus support

This patch adds default Runtime PM callbacks to the dev_pm_ops
belonging to the platform bus. The callbacks are weak symbols
that architecture specific code can override.

Allows Runtime PM even though CONFIG_PM_SLEEP=n.

Signed-off-by: Magnus Damm <damm@igel.co.jp>
Acked-by: Greg Kroah-Hartman <gregkh@suse.de>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>