Make sure we don't suspend if we have no way to wake up the machine.

Make sure we don't suspend if we have no way to wake up the machine.
The idea is that device drivers that can wake up the machine register
themselves by calling device_register_wakeup().  To prevent regressions
on amd64, we let acpi(4) register itself as a wakeup device if the AML
for the machine defines wakeup devices.  This may be refined in the
future.

This diff will prevent people from suspending their arm64 SBCs without
having a way to wake them up.  For now the only driver that registers
itself is axppmic(4), which means that at this moment only SBCs with
an allwinner A64 or H5 SoC will actually support suspend/resume.

ok mlarkin@, deraadt@