Implement MultiwriteNorFlash marker trait on esp-storage::FlashStorage

[yanshay]

Motivations

embedded-srorage defined this trait and various crates are using it (sequential-storage).
w/o it these crates can't be used.
from esp-rs/esp-storage#39 it seems esp32 flash supports the functionality represented by this trait.
I can do that myself and submit a PR but want to verify this is indeed the case.

Solution

Just add the mentioned trait to FlashStorage (when nor-flash feature toggle is enabled)

[bjoernQ]

Yes implementing the MultiwriteNorFlash marker should be fine for all our chips

[yanshay]

Submitted PR #2478