[gta02-core] ECN0010 re-opened (was Re: More audio considerations)

[Werner Almesberger]

I wrote:
> Also, let's try to use IO_3V3 then. As a precaution, I'd add a 0R
> before LDO34IN, so that it can be reworked to use Vsys in case
> there are problems.

Joerg raised a few good points against this plan. He's too shy to
post, so here's my summary:

- current surges would propagate from CODEC_whatever to IO_3V3,
  possibly causing trouble,

- inrush current on LDO3/4 would be drawn from the IO_3V3 capacitors,
  possibly causing trouble as well,

- last but not least, while a DC-DC converter is generally more
  efficient than an LDO, this seems to be a case where they're
  pretty much on par: if Vbat is 3.9 V and we burn 0.6 V in the LDO
  to reach 3.3 V, that's 85% efficiency, just about the same we'd
  expect for the DC-DC converter.

I've removed the section and added a rationale why it's not a great
idea to use IO_3V3 in this case.

- Werner