powerpc/64s: system reset interrupt preserve HSRRs

Code that uses HSRR registers is not required to clear MSR[RI] by
convention, however the system reset NMI itself may use HSRR
registers (e.g., to call OPAL) and clobber them.

Rather than introduce the requirement to clear RI in order to use
HSRRs, have system reset interrupt save and restore HSRRs.

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