MIPS: Loongson64: Drop 32-bit support for Loongson 2E/2F devices

The 32-bit support was broken at runtime, it doesn't boot anymore,
witch is hard to debug because even early printk isn't working, also
there are some build warnings. Some newer bootloader may not support
32-bit ELF. So we decide to drop 32-bit support.

Make loongson64 a pure 64-bit arch.

Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Huacai Chen <chenhc@lemote.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/18174/
Signed-off-by: James Hogan <jhogan@kernel.org>