MAINTAINERS: target/mips: Add MIPS files under default-configs directory

MAINTAINERS: target/mips: Add MIPS files under default-configs directory

Add following files as maintained within the main MIPS target
section in MAINTAINERS:

default-configs/mips64el-linux-user.mak
default-configs/mips64-linux-user.mak
default-configs/mipsn32el-linux-user.mak
default-configs/mipsn32-linux-user.mak
default-configs/mipsel-linux-user.mak
default-configs/mips-linux-user.mak
default-configs/mips64el-softmmu.mak
default-configs/mips64-softmmu.mak
default-configs/mipsel-softmmu.mak
default-configs/mips-softmmu.mak
default-configs/mips-softmmu-common.mak

Future nanoMIPS user mode will also have its .mak file, and
because of that "*mips*" was used instead of "mips*" as a
shorthand in the new item in MAINTAINERS.

Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Aleksandar Markovic <amarkovic@wavecomp.com>