board: starfive: support Milk-V Mars board

board: starfive: support Milk-V Mars board

The differences between the Milk-V Mars board and the VisionFive 2 board
are small enough that we can support both using the same U-Boot build.

* The model and compatible property are taken from proposed Linux patches.
* The EEPROM is atmel,24c02 according to the vendor U-Boot.
* The second Ethernet port is not available.

usb@10100000 does not exist in U-Boot yet. So we don't have to reflect
differences in usage here.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>