Relax spec on 0-bit mux select, use SFC behavior

Change the FIRRTL specification document to allow for 0-bit mux selects.
The existing ZeroWidths pass will promote these to a 1-bit, 0-valued
select signal (which effectively means that the mux can be optimized
away to just the false path).

Signed-off-by: Schuyler Eldridge <schuyler.eldridge@sifive.com>