NMI bit should not be implemented in mie/mip register

Created by: abettati

The Interrupts_t structure is used to define both mie and mip. As it should be used only for hardware error conditions, I think it could be removed from both (as IBEX does). This also follows the specs as they label the upper bits in mie/mip as WPRI fields, meaning future standards can use them. One thing that I don't understand is why IBEX (from which I derived the RI5CY CLINT) uses them for fast lines: looking at the specs upper bits in mie/mip should not be used.