[U-Boot] Improve upstream support for Snowball board (v2)

[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.

v2: Split up networking patch into one for driver and one for board. Fix checkpatch errors. Three warnings are still reported by checkstyle.pl, related to the use of the volatile keyword. These warning are not justified or relevant for MMIO register reads as far as I know, so could be ignored.