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

(cherry picked from commit 665ac052aa04c4dc240d67c3c2166ac63e7f86eb)