stmmac: Add support for SIMATIC IOT2000 platform

The IOT2000 is industrial controller platform, derived from the Intel
Galileo Gen2 board. The variant IOT2020 comes with one LAN port, the
IOT2040 has two of them. They can be told apart based on the board asset
tag in the DMI table.

Based on patch by Sascha Weisenberger.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Sascha Weisenberger <sascha.weisenberger@siemens.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>