MAINTAINERS: Add fw_cfg.c to PPC mac99 machine

MAINTAINERS: Add fw_cfg.c to PPC mac99 machine

The hw/ppc/fw_cfg.c file contains the implementation of
fw_cfg_arch_key_name(), used by the common nvram model. List it under
mac99 machine next to the mac_nvram model.

Cc: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Reviewed-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Signed-off-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>