[gta02-core] More audio considerations

[Werner Almesberger]

I wrote:
> 2) LDO delivers the extra current until thermal shutdown stops it.

I did some measurements on a GTA02v4 with an overload condition on LDO4
(BT_3V2):

State           LDO4 (nominal)	Rload   Ibat (Vbat = 4.2 V)     BT_3V2
--------------- --------------- -----   -------------------     ------
Idle            LDO4 = 0 V      open    104 mA                  3.29 V
Overload        LDO4 = 3.3 V    10 R    422 mA                  3.05 V
Short           LDO4 = 3.3 V     0 R    510 mA                  0.01 V

So the LDO does eventually limit the current, but around 500 mA, which
is considerably above the nominal limit of 150 mA. 

Note that the system didn't reset or crash during this experiment. The
PMU got fairly warm after a while, so I stopped.

- Werner