[gta02-core] rename INT0

[Werner Almesberger]

Dave Ball wrote:
> I've reinstated it to GPA21, and updated ecn0001 to indicate another
> casualty of moving the LCD back home  :-)

Hmm, I think you narrowly avoided one trap but ran into the other :)

All of GPA are referenced to VDDMOP (1.8 V in our case), with the
notable exception of GPA21, which is referenced to VDDOP (3.3 V).
So that's good.

However, the whole GPA block is output-only, so GPA21 can only be
switched between output and nRSTOUT.

- Werner