MIPS: ath79: remove obsolete ATH79_DEV_* configs

Commit 85b9686d ("MIPS: ath79: drop platform device registration code")
removes all files arch/mips/ath79/dev-*.[ch], adjusts the Makefile, but
misses to adjust the Kconfig file. Hence, since then, the configs
ATH79_DEV_* are really dead.

Commit 3a77e0d7 ("MIPS: ath79: drop machfiles") already removes all
configs that select ATH79_DEV_* config. So at that point, they was not a
way to enable them with a kernel build configuration.

Remove these obsolete ATH79_DEV_* configs.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>