powerpc64: make radix with superpages default

As Radix MMU with superpages enabled is now stable, make it the
default choice on supported hardware (POWER9 and above), since its
performance is greater than that of HPT MMU.

Reviewed by: alfredo, jhibbits
Sponsored by: Instituto de Pesquisas Eldorado (eldorado.org.br)
Differential Revision: https://reviews.freebsd.org/D30797