pstore: Support already existing reserved-memory node

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.

Signed-off-by: Detlev Casanova <detlev.casanova@collabora.com>