MAINTAINERS: update MELLANOX HARDWARE PLATFORM SUPPORT maintainers

The "MELLANOX HARDWARE PLATFORM SUPPORT" is maintained as part of the
pdx86 tree. But when Mark and I took over as new pdx86 maintainers the
"MELLANOX HARDWARE PLATFORM SUPPORT" MAINTAINERS entry was not updated.

Update the entry now.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Acked-by: Mark Gross <mgross@linux.intel.com>
Link: https://lore.kernel.org/r/20210216152454.11878-1-hdegoede@redhat.com