Add an isync to after mtsrin, required by the MPC750 errata

MPC750 User Manual Errata (rev 1) adds a note to C.4.2.2 noting that mtsr,
mtsrin, and mtmsr all require a isync after the instruction and before data
address translation uses any of the segment registers. This should make FreeBSD
run correctly on the G3 again.

Reported by: Mark Millard
MFC after: 1 week