MAINTAINERS: Add reviewer for PowerPC TCG CPUs

Nick has great knowledge of the PowerPC CPUs, software and hardware.
Add him as a reviewer on CPU TCG modeling.

Cc: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Cédric Le Goater <clg@kaod.org>

MAINTAINERS

@@ -302,6 +302,7 @@ M: Daniel Henrique Barboza <danielhb413@gmail.com>
 R: Cédric Le Goater <clg@kaod.org>
 R: David Gibson <david@gibson.dropbear.id.au>
 R: Greg Kurz <groug@kaod.org>
+R: Nicholas Piggin <npiggin@gmail.com>
 L: qemu-ppc@nongnu.org
 S: Odd Fixes
 F: target/ppc/