BACKPORT: 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>
(cherry picked from commit 1243cb6a676ffcfa72dd25859edddf66cde0b638)
Signed-off-by: Mark Salyzyn <salyzyn@google.com>
Bug: 154668398
Change-Id: Icc2f0c23da38fa30d667be361792e8cb996ac313