MIPS: Make DIEI support as a config option

DI(Disable Interrupt) and EI(Enable Interrupt) instructions is required by
MIPSR2/MIPSR6, however, it appears to be buggy on some processors such as
Loongson-3A1000. Thus we make it as a config option to allow disable it at
compile time with CPU_MIPSR2 selected.

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