KVM: arm64: timer: Add support for SW-based deactivation

In order to deal with the lack of active state, we need to use
the mask/unmask primitives (after all, the active state is just an
additional mask on top of the normal one).

To avoid adding a bunch of ugly conditionals in the timer and vgic
code, let's use a timer-specific irqdomain to deal with the state
conversion. Yes, this is an unexpected use of irqdomains, but
there is no reason not to be just as creative as the designers
of the HW...

This involves overloading the vcpu_affinity, set_irqchip_state
and eoi callbacks so that the rest of the KVM code can continue
ignoring the oddities of the underlying platform.

Signed-off-by: Marc Zyngier <maz@kernel.org>