platform_bus: allow custom extensions to system PM methods

When runtime PM for platform_bus was added, it allowed for platforms
to customize the runtime PM methods since they are defined as weak
symbols.

This patch allows platforms to also extend the system PM methods with
custom hooks so runtime PM and system PM extensions can be managed
together by custom platform-specific code.

Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>
Cc: Magnus Damm <damm@opensource.se>
Cc: Rafael Wysocki <rjw@sisk.pl>
Cc: Dmitry Torokhov <dtor@mail.ru>
Cc: Eric Miao <eric.y.miao@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>