MIPS: move unselectable entries out of the "CPU type" choice

Move the following entries out of the "CPU type" choice:

 - LOONGSON3_ENHANCEMENT
 - CPU_LOONGSON3_WORKAROUNDS
 - CPU_LOONGSON3_CPUCFG_EMULATION

These entries cannot be selected from the choice because they depend on
CPU_LOONGSON64, which is also located in the same choice.

In fact, Kconfig does not consider them as choice values because they
become children of CPU_LOOONGSON64 due to the automatic submenu creation
in menu_finalize().

However, it is hard to understand this behavior unless you are familiar
with the Kconfig internals.

"choice" should contain only selectable entries.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>