powerpc64: change CAS to support Radix MMU

Use radix_mmu environment variable to select between Hash or Radix
MMU, when performing the CAS method call. This matches kernel's
behavior, by selecting Hash MMU by default and Radix if radix_mmu
is not zero, to make sure that both loader and kernel always select
the same MMU.

The device tree is queried to detect Radix/GTSE support and to
find out if CAS is supported, making the old CPU version and HV
bit checks unnecessary now.

Reviewed by: jhibbits
MFC after: 2 weeks
Sponsored by: Instituto de Pesquisas Eldorado (eldorado.org.br)
Differential Revision: https://reviews.freebsd.org/D31951