Try powering down memory in RP2040 deep sleep

[hierophect]

Section 2.11.4 of the RP2040 datasheet notes that the memory can be set to a low power state during deep sleep using MEMPOWERDOWN. This is not used in the deep sleep examples given by the demo SDK and the amount of power it saves is not described. As noted in #5081, this still does not result in any data loss, it's just a lower power state and apparently needs a little bit of extra time to start back up.

Kind of a low priority feature but it could be considered by anyone who's interested in achieving the lowest possible power for their RP2040 project.