arch-mips: Stop "using namespace std"

arch-mips: Stop "using namespace std"

Change-Id: I0ad5ad71d8ba2d7c050d3f368341ce98d3f87a90
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/39559
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>