docs: powerpc: add elf_hwcaps to table of contents

elf_hwcaps documentation is missing from table of contents at index.rst,
hence triggers Sphinx warning:

Documentation/powerpc/elf_hwcaps.rst: WARNING: document isn't included in any toctree

Add the documentation to the index to fix the warning.

Fixes: 3df1ff42e69e91 ("powerpc: add documentation for HWCAPs")
Reported-by: Stephen Rothwell <[e-mail]>
Signed-off-by: Bagas Sanjaya <[e-mail]>
Signed-off-by: Michael Ellerman <[e-mail]>
Link: https://lore.kernel.org/linuxppc-dev/[e-mail]/
Link: https://lore.kernel.org/r/[e-mail]

Documentation/powerpc/index.rst

···
     dawr-power9
     dscr
     eeh-pci-error-recovery
+    elf_hwcaps
     elfnote
     firmware-assisted-dump
     hvcs