mn10300: Use generic idle loop

mn10300: Use generic idle loop

default_idle() boils down to safe_halt() and the other option is idle
polling. Use the core functionality.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: Paul McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Reviewed-by: Cc: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
Cc: Magnus Damm <magnus.damm@gmail.com>
Cc: David Howells <dhowells@redhat.com>
Link: http://lkml.kernel.org/r/20130321215234.822656036@linutronix.de
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>