target/ppc: Implement core timebase state machine and TFMR

target/ppc: Implement core timebase state machine and TFMR

This implements the core timebase state machine, which is the core side
of the time-of-day system in POWER processors. This facility is operated
by control fields in the TFMR register, which also contains status
fields.

The core timebase interacts with the chiptod hardware, primarily to
receive TOD updates, to synchronise timebase with other cores. This
model does not actually update TB values with TOD or updates received
from the chiptod, as timebases are always synchronised. It does step
through the states required to perform the update.

There are several asynchronous state transitions. These are modelled
using using mfTFMR to drive state changes, because it is expected that
firmware poll the register to wait for those states. This is good enough
to test basic firmware behaviour without adding real timers. The values
chosen are arbitrary.

Acked-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>