Only crop the VPN on POWER4 and derivatives for TLBIE operations

Summary:
PowerISA 2.03 and later require bits 14:65 in the RB register argument,
which is the full value of the vpn argument post-shift. Only POWER4, POWER4+,
and PPC970* need the upper 16 bits cropped.

With this change FreeBSD can boot to multi-user on POWER9.

Reviewed by: nwhitehorn
Differential Revision: https://reviews.freebsd.org/D15581