pstore: Do not fail if a reserved-memory node already exists

The pstore command tries to create a reserved-memory node but fails if it is already present with:

Add 'reserved-memory' node failed: FDT_ERR_EXISTS

This patch creates the node only if it does not exist and adapts the reg values sizes depending on already present #address-cells and #size-cells values.

https://phabricator.apertis.org/T8435