powerpc: Update CPUTYPE options in make.conf

Document the supported PPC/POWER CPUTYPEs in the example make.conf.

Update bsd.cpu.mk to handle CPUs newer than POWER9, and remove
32-bit CPUTYPEs since we no longer support those at all.

Reviewed by:	imp, jhibbits
Sponsored by:	https://www.patreon.com/bsdivy
Differential Revision:	https://reviews.freebsd.org/D54257

share/examples/etc/make.conf

@@ -47,7 +47,12 @@
 #			cortex-a7, cortex-a8, cortex-a9, cortex-a12,
 #			cortex-a15, cortex-a17
 #   ARM64 architecture:	cortex-a53, cortex-a57, cortex-a72,
-#				exynos-m1
+#			exynos-m1
+#   POWER / PowerPC architecture:
+#       (POWER CPUs)	power5, power5x, power6, power6x, power7, power8,
+#			power9, power10, power11
+#       (PowerPC CPUs)	g5, 970, e5500
+#
 #
 # (?= allows to buildworld for a different CPUTYPE.)
 #