Add fastboot oem commands to access SPMI/PMIC

[fxsheep]

Usage:

fastboot oem spmi read <address>
fastboot oem spmi write <address> <value>

[fxsheep]

Do note that this two commands (among many other lk2nd debugging-orientated features) give the possibility to severely and permanently damage the hardware (e.g. one wrong voltage and half of motherboard is fried). IMHO lk2nd lacks a warning/precaution of such a risk.

[TravMurav]

Hi, We are considering to obsolete the current lk2nd branch in favor of the different one (currently in experimental-tmp) in the near future, so I'd appreciate if you could target that with your PR. This should be pretty easy, please see e.g. https://github.com/msm8916-mainline/lk2nd/blob/experimental-tmp/lk2nd/fastboot/misc.c

The file will need to be conditionally compiled for those targets that have the pmic.

[fxsheep]

Thanks for clarification and advice. I'm closing this temporarily.

[stephan-gh]

I'd still like to keep this open as a reminder. Basically the idea is to have a lk2nd/fastboot/debug module in the new branch which would implement functionality such as the readl/writel or something like this.

I suppose we could disable it by default if it's considered too dangerous (although generally I'd hope users are reasonable enough to avoid using commands they do not understand).

[stephan-gh]

I added fastboot oem debug readpmic and fastboot oem debug writepmic in the experimental-tmp3 branch of the new lk2nd rebase. Thanks for the idea!