mie.LCOFIE is not implemented correctly #246

Bug Description:

Bit 13 of the mie register is LCOFIE.

The RISCV specification is as follows:

If the Sscofpmf extension is implemented, bits mip.LCOFIP and mie.LCOFIE are the interrupt-pending
and interrupt-enable bits for local-counter-overflow interrupts. LCOFIP is read-write in mip and reflects
the occurrence of a local counter-overflow overflow interrupt request resulting from any of the
mhpmeventn.OF bits being set. If the Sscofpmf extension is not implemented, mip.LCOFIP and
mie.LCOFIE are read-only zeros.

The NutShell processor does not implement the Sscofpmf extension, therefore bit 13 of the mie should be read-only zero. Furthermore, testing confirmed that mip.LCOFIP is implemented correctly.

Test program and log information: test_mie.zip

Inconsistent information:

Environment:

NutShell: commit e315a27