siemens: board: etamin: remove deprecated nand config

NAND was used in the early development phase of etamin. The board runs now
on MMC. This setting is no more used -> remove to simplify the board file.

Further clean-up of etamin should remove all NAND settings. Complete clean-
up of etamin board will take place in a separate patch serie.

Signed-off-by: Enrico Leto <enrico.leto@siemens.com>