KVM: arm64: nv: Add basic emulation of AT S1E2{R,W}

Similar to our AT S1E{0,1} emulation, we implement the AT S1E2
handling.

This emulation of course suffers from the same problems, but is
somehow simpler due to the lack of PAN2 and the fact that we are
guaranteed to execute it from the correct context.

Co-developed-by: Jintack Lim <jintack.lim@linaro.org>
Signed-off-by: Jintack Lim <jintack.lim@linaro.org>
Signed-off-by: Marc Zyngier <maz@kernel.org>