ASoC: Add stub suspend and resume calls for ASoC subdevices

Now that ASoC subdevices can be regular devices they can have normal
suspend and resume calls from their buses.  However, suspending them
individually is not desirable since this can lead to problems such as
pops and clicks from devices being suspended with their signals being
amplified or clocks being stopped suddenly.

This will be resolved by having the normal device model suspend and
resume calls call into ASoC which will suspend the entire card while any
of its components are suspended.  At present this is not yet implemented
but in order to aid the transition of drivers to the standard device
model this patch adds API calls for the notifications.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>