KVM: arm64: pkvm: Handle GICv3 traps as required

Forward accesses to the ICV_*SGI*_EL1 registers to EL1, and
emulate ICV_SRE_EL1 by returning a fixed value.

This should be enough to support GICv3 in a protected guest.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Fuad Tabba <tabba@google.com>
Tested-by: Fuad Tabba <tabba@google.com>
Link: https://lore.kernel.org/r/20211013120346.2926621-7-maz@kernel.org