brcm63xx: fix HG556a boards and remove fallback board

This changes board info to match the Wiki
http://wiki.openwrt.org/toh/huawei/hg556a

, removes the no longer needed
fallback board and fixes HG556 Ralink eeprom extraction.

Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>
[jogo: rename partitions instead]
Signed-off-by: Jonas Gorski <jogo@openwrt.org>

SVN-Revision: 42206