arch-mips: Replaced `BigEndianByteOrder` in MIPS

arch-mips: Replaced `BigEndianByteOrder` in MIPS

The following change removed the `BigEndianByteOrder` enum and replaced
it with `ByteOrder:big`:
https://gem5-review.googlesource.com/c/public/gem5/+/33174

This change was not propogated to `src/arch/mips/isa/decoder.isa` and
`src/arch/mips/isa/formats/mem.isa`, and therefore caused compilation
errors. This caused the Nightly Build to fail:
https://www.mail-archive.com/gem5-dev@gem5.org/msg35900.html

This commit fixes this error.

Change-Id: I3967eb9e9236a7a95318c17ca410b613b8473eed
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/34255
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>