arm64: add kdump.rst into index.rst

Document kdump.rst was added into Documentation/arm64/, but not listed
in Documentation/arm64/index.rst. That triggers below warning when
executing "make htmldoc":

>> Documentation/arm64/kdump.rst: WARNING: document isn't included in any toctree

Fix it now.

Reported-by: kernel test robot <[email protected]>
Closes: https://lore.kernel.org/oe-kbuild-all/[email protected]/
Signed-off-by: Baoquan He <[email protected]>
Fixes: 03dc0e0 ("Documentation: add kdump.rst to present crashkernel reservation on arm64")
Reviewed-by: Bagas Sanjaya <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Catalin Marinas <[email protected]>

Author: Baoquan He

Documentation/arm64/index.rst

@@ -15,6 +15,7 @@ ARM64 Architecture
     cpu-feature-registers
     elf_hwcaps
     hugetlbpage
+    kdump
     legacy_instructions
     memory
     memory-tagging-extension