arm64: timer: Use correct counter for !pending_before

We were using the virtual counter to calculate a timer cval which is 10
seconds in the future, but this obviously doesn't work for the physical
timer which is bases on the physical counter.

Make sure we use a properly paired timer/counter pair.

Reported-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Christoffer Dall <cdall@linaro.org>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>