board-pico.h: allow to rework memory a *little* (experimental)

- round off memory allocations by default (shouldn't make much of a difference)
- add *experimental* allocation values!
- add some commenting (will be useful later)
defconfig: Enable expanded RAM
i2c: tps65200: Use fast charge rate when slow charge is requested

For AC-charge rate over USB.

Signed-off-by: Sultanxda <sultanxda@gmail.com>
Set ION heap sizes in separate function