ramips: add image for ALL0256N-8M

The new version of the ALL0256N now got 8MB SPI NOR flash instead of 4MB.
In order to expose the whole amount of flash, add another image which contains
the corresponding GENERIC_8M mtdparts.

[juhosg: change suffix from '8m' to '8M']

Patch by: Daniel Golle <dgolle@allnet.de>

SVN-Revision: 33614