Limited Ethernet speed to 10/100 Mbps

Due to a problem in U-MoBo schematics, the board does not work against Gigabit Ethernet switches, when mounting an i.MX6 SoM.

This change to U-Boot support for silverbullet prevent Micrel KSZ9031 to advertise its Gigabit capabilities, allowing the board to work with Gigabit switches at reduced speed (10/100 MBps).

Since the hardware does not work at all when connected to Gigabit switches, then I think that this change *MUST* be merged into the original u-mobo/uboot-imx repository; the only other option is to modify the hardware, but we cannot do that on GitHub ;) and anyway it won't fix the boards already produced.