MAINTAINERS: Add reviewers for PowerNV baremetal emulation

Fred and Nick have been hacking baremetal POWER systems (OPAL) for
many years. They use and modify the QEMU models regularly. Add them as
PowerNV reviewers.

Cc: Frédéric Barrat <fbarrat@linux.ibm.com>
Cc: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Cédric Le Goater <clg@kaod.org>
Acked-by: Frederic Barrat <fbarrat@linux.ibm.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Cédric Le Goater <clg@kaod.org>

MAINTAINERS

@@ -1451,6 +1451,8 @@ F: tests/avocado/ppc_pseries.py
 
 PowerNV (Non-Virtualized)
 M: Cédric Le Goater <clg@kaod.org>
+R: Frédéric Barrat <fbarrat@linux.ibm.com>
+R: Nicholas Piggin <npiggin@gmail.com>
 L: qemu-ppc@nongnu.org
 S: Odd Fixes
 F: docs/system/ppc/powernv.rst