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 <sfr@canb.auug.org.au>
Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/linuxppc-dev/20220727220050.549db613@canb.auug.org.au/
Link: https://lore.kernel.org/r/20220728033332.27836-4-bagasdotme@gmail.com