openrisc: Initial support for the idle state

This patch adds basic support for the idle state of the cpu.
The patch overrides the regular idle function, enables the interupts,
checks for the power management unit and enables the cpu doze mode
if available.

Signed-off-by: Sebastian Macke <sebastian@macke.de>
[shorne@gmail.com: Fixed checkpatch, blankline after declarations]
Signed-off-by: Stafford Horne <shorne@gmail.com>