KVM: arm/arm64: arch_timer: Fix CNTP_TVAL calculation

Recently the generic timer test of kvm-unit-tests failed to complete
(stalled) when a physical timer is being used. This issue is caused
by incorrect update of CNTP_CVAL when CNTP_TVAL is being accessed,
introduced by 'Commit 84135d3d ("KVM: arm/arm64: consolidate arch
timer trap handlers")'. According to Arm ARM, the read/write behavior
of accesses to the TVAL registers is expected to be:

  * READ: TimerValue = (CompareValue – (Counter - Offset)
  * WRITE: CompareValue = ((Counter - Offset) + Sign(TimerValue)

This patch fixes the TVAL read/write code path according to the
specification.

Fixes: 84135d3d ("KVM: arm/arm64: consolidate arch timer trap handlers")
Signed-off-by: Wei Huang <wei@redhat.com>
[maz: commit message tidy-up]
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>