KVM: arm64: Remove PMSWINC_EL0 shadow register

We keep an entry for the PMSWINC_EL0 register in the vcpu structure,
while *never* writing anything there outside of reset.

Given that the register is defined as write-only, that we always
trap when this register is accessed, there is little point in saving
anything anyway.

Get rid of the entry, and save a mighty 8 bytes per vcpu structure.

We still need to keep it exposed to userspace in order to preserve
backward compatibility with previously saved VMs. Since userspace
cannot expect any effect of writing to PMSWINC_EL0, treat the
register as RAZ/WI for the purpose of userspace access.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20210719123902.1493805-5-maz@kernel.org