musb_hdrc: Add board specific external clock handling

On N800, if osc_ck is disabled by PM, USB will stop working.

Signed-off-by: Tony Lindgren <tony@atomide.com>
Updated so some of this code can be shared with other HDRC implementations;
pretty much everything except the TUSB 6010 EVM needs a clock.  This will
need more work in the future.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>