MAINTAINERS: Include mlxreg.h in Mellanox Platform Driver files

Avoid conflicts from other subsystems by including the header with the
rest of the driver files.

Cc: Andy Shevchenko <andy@infradead.org>
Cc: Vadim Pasternak <vadimp@mellanox.com>
Acked-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Darren Hart (VMware) <dvhart@infradead.org>