The reason is described in 5033ad5660 ("MIPS: move unselectable
entries out of the "CPU type" choice").
At the same time, commit 101bd58fde ("MIPS: Add support for Mobileye
EyeQ5") introduced another unselectable choice member.
(In fact, 5033ad5660 and 101bd58fde have the same commit time.)
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>