boards: nrf52840dongle_nrf52840: restore storage partition

This restores the 'storage' partition (16 KiB) in the stock layout
(fstab-stock.dts), removed in 9a842588, probably by mistake (the
commit description refers to scratch area only).

This was found when building OpenThread Co-Processor sample.

Fixes #53689

Signed-off-by: Piotr Dymacz <pepe2k@gmail.com>