[DEBUG] KVM: arm64: Add debug UART hacks at EL2

Provide a standalone mechanism for debug UART access at EL2. It's a
debug hack, so please try not to get too offended by it.

[ qperret: re-map UART at EL2 in protected mode ]

Signed-off-by: Will Deacon <will@kernel.org>
Signed-off-by: Quentin Perret <qperret@google.com>
[ Rebase onto v6.6 ]
Signed-off-by: James Clark <james.clark@arm.com>