xen/arm: remove physical timer offset

xen/arm: remove physical timer offset

The physical timer traps apply an offset so that time starts at 0 for
the guest. However, this offset is not currently applied to the physical
counter. Per the ARMv8 Reference Manual (ARM DDI 0487E.a), section
D11.2.4 Timers, the "Offset" between the counter and timer should be
zero for a physical timer. This removes the offset to make the timer and
counter consistent.

This also cleans up the physical timer implementation to better match
the virtual timer - both cval's now hold the hardware value.

In the case the guest sets cval to a time before Xen started, the correct
behavior is to expire the timer immediately. To do this, we set the expires
argument of set_timer to zero.

Signed-off-by: Jeff Kubascik <jeff.kubascik@dornerworks.com>
Acked-by: Julien Grall <julien@xen.org>