Initialize MTIP at reset

Since mtime and mtimecmp both initialize to 0, and "a machine timer
interrupt becomes pending whenever mtime contains a value greater than
or equal to mtimecmp," MTIP should be 1.

Before this change, MTIP would be 0 until the first increment, or
until reading/writing from CLINT regs (since clint_t::load() and
clint_t::store() call increment(0)).