MAINTAINERS: Update file items for MIPS Malta board

hw/mips/gt64xxx_pci.c is used for Malta only, so it is logical to
place this file in Malta board section of the MAINTAINERS file.

Signed-off-by: Aleksandar Markovic <amarkovic@wavecomp.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Aleksandar Rikalo <arikalo@wavecomp.com>
Message-Id: <1561037595-14413-2-git-send-email-aleksandar.markovic@rt-rk.com>

MAINTAINERS

@@ -930,6 +930,7 @@ M: Aurelien Jarno <aurelien@aurel32.net>
 R: Aleksandar Rikalo <arikalo@wavecomp.com>
 S: Maintained
 F: hw/mips/mips_malta.c
+F: hw/mips/gt64xxx_pci.c
 F: tests/acceptance/linux_ssh_mips_malta.py
 
 Mipssim