openrisc: Allow power off handler overriding

The OpenRISC platform always defines a default pm_power_off hanlder
which is only useful for simulators.  Having this set also means power
management drivers like syscon-power are not able to wire in their own
pm_power_off handlers.

Fix this by not setting the pm_power_off handler by default and fallback
to the simulator power off handler if no handler is set.

This has been tested with a new OpenRISC virt platform I am working on
for QEMU.

  https://github.com/stffrdhrn/qemu/commits/or1k-virt



Cc: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Stafford Horne <shorne@gmail.com>