powerpc: add documentation for HWCAPs

Take the arm64 HWCAP documentation file and adjust it for powerpc.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Reviewed-by: Segher Boessenkool <segher@kernel.crashing.org>
[mpe: Fix ARCH_2_05 comment, as noticed by Tulio.]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220715012636.165948-1-npiggin@gmail.com