The reason is described in 5033ad56 ("MIPS: move unselectable
entries out of the "CPU type" choice").

At the same time, commit 101bd58f ("MIPS: Add support for Mobileye
EyeQ5") introduced another unselectable choice member.

(In fact, 5033ad56 and 101bd58f have the same commit time.)
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>