MIPS: Loongson64: env: Hook up Loongsson-2K

Somehow those enablement bits were left over when we were
adding initial Loongson-2K support.

Set up basic information and select proper builtin DTB for
Loongson-2K.

Cc: stable@vger.kernel.org
Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>