KVM: arm64: Make kvm_at() take an OP_AT_*

To allow using newer instructions that current assemblers don't know about,
replace the `at` instruction with the underlying SYS instruction.

Signed-off-by: Joey Gouly <joey.gouly@arm.com>
Cc: Oliver Upton <oliver.upton@linux.dev>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Reviewed-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Anshuman Khandual <anshuman.khandual@arm.com>
Acked-by: Will Deacon <will@kernel.org>
Signed-off-by: Marc Zyngier <maz@kernel.org>