shmobile: Use power domains for platform runtime PM

shmobile platforms replace the runtime PM callbacks of the platform
bus type with their own routines, but this means that the callbacks
are replaced system-wide.  This may not be the right approach if the
platform devices on the system are not of the same type (e.g. some
of them belong to an SoC and the others are located in separate
chips), because in those cases they may require different handling.
Thus it is better to use power domains to override the platform bus
type's PM handling, as it generally is possible to use different
power domains for devices with different PM requirements.

Define a default power domain for shmobile in both the SH and ARM
falvors and use it to override the platform bus type's PM callbacks.
Since the suspend and hibernate callbacks of the new "default" power
domains need to be the same and the platform bus type's suspend and
hibernate callbacks for the time being, export those callbacks so
that can be used outside of the platform bus type code.

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>