[U-Boot] Marvell U-Boot co-custodian

Hi Tom, Hi Wolfgang,

I've talked with Luka and he agrees, that it makes sense for me to become a co-custodian for the Marvell U-Boot git repository.

So, if nobody objects, Wolfgang, could you please add my ssh key to the Marvell U-Boot git repository "u-boot-marvell"?

Thanks, Stefan