powerpc/64s/kuap: Restore AMR in system reset exception

The system reset interrupt handler locks AMR and exits with
EXCEPTION_RESTORE_REGS without restoring AMR. Similarly to the
soft-NMI handler, it needs to restore.

Fixes: 890274c2

 ("powerpc/64s: Implement KUAP for Radix MMU")
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200429065654.1677541-5-npiggin@gmail.com