Add ePAPR boot support for PowerPC book-E (MPC85xx) hardware

Summary:
u-boot, following the ePAPR specification, puts secondary cores into a
spinloop at boot, rather than leaving them shut off. It then relies on the host
OS to write the correct values to a special spin table, located in coherent
memory (on newer implementations), or noncoherent memory (older
implementations).

This supports both implementations of ePAPR, as well as continuing to support
non-ePAPR booting, by first attempting to use the spintable, and falling back to
expecting non-started CPUs.

Test Plan:
Booted on a P5020 board. Tested before and after the changes.
Before the changes, prints the error "SMP: CPU 1 already out of hold-off state!"
and panics shortly thereafter. After the changes, same boot method lets it
complete boot.

Reviewed by: nwhitehorn
MFC after: 2 weeks
Relnotes: Yes
Sponsored by: Alex Perez/Inertial Computing
Differential Revision: https://reviews.freebsd.org/D7494