KVM: arm64: Use visibility hook to treat ID regs as RAZ

The generic id reg accessors already handle RAZ registers by way of the
visibility hook. Add a visibility hook that returns REG_RAZ
unconditionally and throw out the RAZ specific accessors.

Reviewed-by: Reiji Watanabe <reijiw@google.com>
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20220913094441.3957645-2-oliver.upton@linux.dev