On the face of it, AT S12E{0,1}{R,W} is pretty simple. It is the
combination of AT S1E{0,1}{R,W}, followed by an extra S2 walk.

However, there is a great deal of complexity coming from combining
the S1 and S2 attributes to report something consistent in PAR_EL1.

This is an absolute mine field, and I have a splitting headache.
Signed-off-by: Marc Zyngier <maz@kernel.org>