powerpc/64s: Fix FIXUP_ENDIAN non-maskable interrupt reentrancy

FIXUP_ENDIAN uses SRR[01] with MSR_RI=1, which gets corrupted if there
is an interleaving system reset or machine check interrupt.

Set MSR_RI=0 before setting SRRs. The rfid will restore MSR.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>