gen/genlib/misc: Add CE option to WaitTimer

[rowanG077]

One nice thing this allows is pipelining and combining of multiple timers. For example imagine you need to have multiple timers that are relatively long. One option now is to create a base timer and then on every base timer done use that as the CE of the other timers.

This is now possible:

self.timer1 = timer1 = WaitTimer(10)
self.timer2 = timer2 = WaitTimer(5, has_ce=True)
self.timer3 = timer3 = WaitTimer(3, has_ce=True)

self.comb += [
    self.timer1.wait.eq(~self.timer1.done),
    self.timer2.wait.eq(~self.timer2.done),
    self.timer3.wait.eq(~self.timer3.done),
]

self.sync += [
    self.timer2.ce.eq(self.timer1.done),
    self.timer3.ce.eq(self.timer1.done),
]

timer2 will now done every 5*10 cycles and timer3 is now done every 3*10 cycles