Support guest access to the physical timer when FEAT_ECV_POFF is
supported. In this case we can set an offset for the physical timer.
We can reuse the virtual timer support to also support the physical
timer, with a few more registers needing to be handled when switching
to a guest.

As it is not clear how this will affect performance when the guest
doesn't use it hide enabling it behind a sysctl.

It is expected this will be useful when Nested Virtualisation is
supported as guests are expected to use the physical timer registers.

Sponsored by: Arm Ltd