mips: remove reference to "newer Loongson-3"

Newest Loongson-3 processors have moved to use LoongArch architecture.
Sadly, the LL/SC issue is still existing on both latest Loongson-3
processors using MIPS64 (Loongson-3A4000) and LoongArch
(Loongson-3A5000).

As it's very unlikely there will be new Loongson-3 processors using
MIPS64, let's stop people from false hoping.

Signed-off-by: Xi Ruoyao <xry111@mengyan1223.wang>
Cc: Huacai Chen <chenhuacai@kernel.org>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>