Don't call device_set_ivars() for the mmchs, it doesn't seem to be used,
and it overrides the ivars set by the simplebus.
Change the way pcpu and curthread are stored per-core:
the old way was to store pcpu in a register, and get curthread from pcpu,
which is not very atomic, and led to issues if the thread was migrated
to another core between the time we got the pcpu address and the time we
got curthread.
Instead, we now store curthread where pcpu used to be store, and we
calculate the pcpu address based on the cpu id.
o Add prototype for tcon_bypass() used by dcu4
o Add register definition