MAINTAINERS: Update Mellanox and Cumulus Network addresses to new domain

Mellanox and Cumulus Network were acquired by Nvidia, so change the
maintainers emails to new domain name.

Link: https://lore.kernel.org/r/20200810091100.243932-1-leon@kernel.org
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Acked-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>