MIPS: Loongson32: Revert ISA level to MIPS32R2

GS232 core have implemented all necessary mips32r2 instructions.
Serval missing FP instructions can be emulated by kernel.

The issue of di instruction have been solved.
Thus we revert the ISA level back to MIPS32R2.

Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Signed-off-by: Paul Burton <paul.burton@mips.com>
Cc: linux-mips@vger.kernel.org
Cc: keguang.zhang@gmail.com