arm64: Add tagged-address-abi.rst to index.rst

Documentation/arm64/tagged-address-abi.rst introduces the
relaxation of the syscall ABI that allows userspace to pass
certain tagged pointers to kernel syscalls.

Add the document to index.rst for a correct generation of the
table of content.

Cc: Will Deacon <will@kernel.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>