u-boot fix to charge empty battery

[Philipp Hug]

I played around with u-boot and I think I found a solution to the
problem, that u-boot doesn't load when the battery is too empty.
The reason seems to be this line:
       [PCF50633_REG_BVMCTL]   = 0x02, /* 2.80V BAT voltage thresh. */

This causes the power management module to shut down the phone before it
starts charging.
I fixed this and some more power related stuff.

Have a look here: (there's also a flashable u-boot image)
http://www.abanet.ch/~hug/moko/uboot-battery/

Feedback is welcome.

Philipp