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 1243cb6a67)
Signed-off-by: Mark Salyzyn <salyzyn@google.com>
Bug: 154668398
Change-Id: Icc2f0c23da38fa30d667be361792e8cb996ac313

Documentation/arm64/index.rst

@@ -0,0 +1,27 @@
+==================
+ARM64 Architecture
+==================
+
+.. toctree::
+    :maxdepth: 1
+
+    acpi_object_usage
+    arm-acpi
+    booting
+    cpu-feature-registers
+    elf_hwcaps
+    hugetlbpage
+    legacy_instructions
+    memory
+    pointer-authentication
+    silicon-errata
+    sve
+    tagged-address-abi
+    tagged-pointers
+
+.. only::  subproject and html
+
+   Indices
+   =======
+
+   * :ref:`genindex`