Add code to identify the CPUs on arm64 systems. The primary CPU is attached

Add code to identify the CPUs on arm64 systems.  The primary CPU is attached
and identified early on.  For the secondary CPUs this happens late, such that
the drivers we need to spin up CPUs, such as psci(4), will be available.

This also fixes some code in simplebus(4) where the return value of
OF_getprop() was not properly checked.

Heavily based on an earlier diff from drahn@.

ok drahn@, jsg@