arm64: Fix dangling references to Documentation/arm64

The arm64 documentation has moved under Documentation/arch/; fix up
references in the arm64 subtree to match.

Cc: Will Deacon <will@kernel.org>
Cc: Ard Biesheuvel <ardb@kernel.org>
Cc: linux-efi@vger.kernel.org
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>