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 <[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/elf_hwcaps.rst

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