clicinttrig register interrupt_number should be 12 bits wide

[mark-honman]

In section 3.6 figure 2 shows interrupt_nunber in the clicinttrig register occupying bits 12 down to 0 (13 bits), while the 4096 interrupt sources can be encoded in 12 bits.

Also... this is the only place in the spec where "interrupt number" is used, elsewhere the term used is "interrupt ID" or "interrupt code".