OMAP3: move USBHOST SAR handling from clock framework to powerdomain layer

Remove usbhost_sar_fclk from the OMAP3 clock framework.  The bit that
the clock was tweaking doesn't actually enable or disable a clock; it
controls whether the hardware will save and restore USBHOST state
when the powerdomain changes state.  (That happens to coincidentally
enable a clock for the duration of the operation, hence the earlier
confusion.)

In place of the clock, mark the USBHOST powerdomain as supporting
hardware save-and-restore functionality.

Signed-off-by: Paul Walmsley <paul@pwsan.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>