mstatus.MPP should not be 2

[fly-1011]

Bug Description:

Hello, I found that mstatus.MPP in NutShell can be 2.

Inconsistent information:

Bit 12 in mstatus is different. According to the riscv specification, bits 11 and 12 are the MPP field. The MPP field in NutShell is 10. A valid write check should be added to mstatus.mpp.

Test program and log information: test_MPP.zip

./build/emu -b 0 -e 0 -i test_MPP.img --diff /xs-env/ready-to-run/riscv64-nutshell-spike-so

Environment:
NutShell: commit e315a27