u-boot low-battery handling

[Andy Green]

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Somebody in the thread at some point said:

| I haven't looked at Qi yet - maybe that's a project for next weekend.

For these purposes, it's U-Boot with almost all power management ripped out.

| I don't know if you'll be able to put all of the PMU control logic into
| Linux. It seems that it might still make sense to have code in the
| earliest bootloader for cases like deeply-discharged batteries (e.g.
| turn on the charger, flash an LED, and wait for the voltage to reach
| some threshold before powering up the glamo, SD card, and whatever else
| is needed to boot Linux).

What I am hoping we can do here is boot at slow CPU for the initial part
of Linux boot to always be below 500mW on average, and stall in pcf50633
driver init (which will be early in boot with 2.6.26 soon) until we
either detect we have no charger, 1A charger, or get enumeration from
host PC and can use 500mA ower.  Typically this will add a second or so.

- -Andy
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