docs: powerpc: use different label name for elf_hwcaps.rst

Sphinx reported duplicate label warning:

WARNING: duplicate label elf_hwcaps_index, other instance in Documentation/arm64/elf_hwcaps.rst

The warning is caused by elf_hwcaps_index label name is already used for
arm64 documentation, whileas powerpc use the same name.

Disambiguate the label name for powerpc.

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/20220728033332.27836-3-bagasdotme@gmail.com

Documentation/powerpc/elf_hwcaps.rst

-.. _elf_hwcaps_index:
+.. _elf_hwcaps_powerpc:
 
 ==================
 POWERPC ELF HWCAPs