MAINTAINERS: Add three files to Malta section

Add three files that were recently introduced in a refactoring,
that Malta emulation relies on. They are added by this patch
to Malta section, but they are not added to the general MIPS
section, since they are really not MIPS-specific, and there
may be some non-MIPS hardware using them in future.

Signed-off-by: Aleksandar Markovic <amarkovic@wavecomp.com>
Acked-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <1575982519-29852-5-git-send-email-aleksandar.markovic@rt-rk.com>

MAINTAINERS

@@ -959,8 +959,11 @@ M: Aleksandar Markovic <amarkovic@wavecomp.com>
 M: Philippe Mathieu-Daudé <f4bug@amsat.org>
 R: Aurelien Jarno <aurelien@aurel32.net>
 S: Maintained
+F: hw/isa/piix4.c
+F: hw/acpi/piix4.c
 F: hw/mips/mips_malta.c
 F: hw/mips/gt64xxx_pci.c
+F: include/hw/southbridge/piix.h
 F: tests/acceptance/linux_ssh_mips_malta.py
 
 Mipssim