Add submodules = True to MOM5 SPD

[dougiesquire]

ACCESS-NRI/MOM5#27 adds a git submodule to MOM5. This is hopefully temporary while @harshula works on a better solution. But it would good to be able to build and test the code changes in the meantime. To do so requires adding submodules = True to the MOM5 SPD.

I'll follow up with a PR.