Add some Muen specialities

This is early WIP. Currently, we get a correct wallclock, and disable
periodic I/O port accesses for the PIT and PIC.

o A clocksource driver reading from a shared memory word (at 0x1000)
  This could be done in a module (as a guest-tool).

o A no-op clockevent PIT replacement

o Register some no-op functions for PIC IRQ masking / EOI