sha/asm/sha512p8-ppc.pl: add POWER8 performance data.

[skip ci]
Reviewed-by: NBernd Edlinger <bernd.edlinger@hotmail.de>
Reviewed-by: NRich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/3705)

crypto/sha/asm/sha512p8-ppc.pl

@@ -30,6 +30,15 @@
 # for sha1-ppc.pl - 73%. 100% means that multi-process result equals
 # to single-process one, given that all threads end up on the same
 # physical core.
+#
+######################################################################
+# Believed-to-be-accurate results in cycles per processed byte [on
+# little-endian system]. Numbers in square brackets are for 64-bit
+# build of sha512-ppc.pl, presented for reference.
+#
+#		POWER8
+# SHA256	9.9 [15.8]
+# SHA512	6.3 [10.3]
 
 $flavour=shift;
 $output =shift;