arm64: Update Documentation/arm references

The Arm documentation has moved to Documentation/arch/arm; update
references under arch/arm64 to match.

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