nios2: implement nano_cpu_idle and nano_cpu_atomic_idle

Nios II does not have a power saving instruction, so there is nothing
to really do here other than ensure that interrupts are in the correct
state when leaving nano_cpu_atomic_idle().

Change-Id: I664c7542dc2fc1795a453d35e183a737dcb20c38
Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>