Add sh7372 specific code to power down unused pm domains.

This should really be replaced by some generic PM core
code IMO, but until that happens this patch makes sure
we don't waste power by leaving unused power domains on.
Signed-off-by: NMagnus Damm <damm@opensource.se>
Signed-off-by: NRafael J. Wysocki <rjw@sisk.pl>