AMD Geode cpus: apply un-written naming rules

AMD Geode cpus: apply un-written naming rules

Kconfig directives to select chip drivers for compile literally
match the chip directory names capitalized and underscored.

Rename directories and Kconfig as follows:
   model_lx  -> geode_lx
   model_gx1 -> geode_gx1
   model_gx2 -> geode_gx2

Change-Id: Ib8bf1e758b88f9efed1cf8b11c76b796388e7147
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: http://review.coreboot.org/613
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>