storage: flash_map: Fix partition device retrieval

Change the default behaviour of the flash partition choice when no
matching compatible is found. The new default is the grandparent node
of the fixed-partition.

As the grandparent node is what the jedec_spi_nor and nordic_qspi_nor
compatibles were selecting, remove these as separate checks.

This allows the flash_map API to be used for implementations of the
flash API other than the 3 originally specified (soc_nv_flash,
jedec_spi_nor & nordic_qspi_nor).

Fixes #26397

Signed-off-by: Jordan Yates <jordan.yates@data61.csiro.au>