[U-Boot] Improve upstream support for Snowball board

[Roy Spliet]

Patches are loosely based on the tree maintained by Calao. FDT confirmed working, LAN is successfully detected by u-boot (but lacking a proper tftp setup this is untested). Upstream U-Boot still has one bug that prevents booting from eMMC. Easiest workaround is to revert

e95504497ecac46907204b0ee3460b708a2981ac mmc: Split device init to decouple OCR-polling delay 

until I (or someone else) has time to device a proper fix. This doesn't prevent anyone from pushing these two patches though.