x86, mrst: Some drivers need to known when an SCU is available

Add a notifier so that drivers can hook into SCU availability in order to
take actions post initialisation when/if the SCU becomes available.

In the ideal world we wouldn't need this and we could avoid any init
dependancies of this form, but in practice we can't do it for some cases.

Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>