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

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

Use HSRR[01] instead, which does not require MSR_RI=0.

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