devicetree: Add DT_FIXED_PARTITION_ADDR macro

This convenience API returns the absolute address of a fixed partition,
i.e., relative offset + base address. It's distinct from `DT_REG_ADDR()`
and `FIXED_PARTITION_OFFSET()`, both of which return just the offset.

The base address is taken from the parent memory node as given by the
newly added `DT_MEM_FROM_FIXED_PARTITION()`. This is expected to ensure
that the returned address is directly addressable by the CPU. This is
also meant to prevent `DT_FIXED_PARTITION_ADDR()` from working with
external memory partitions.

Signed-off-by: Grzegorz Swiderski <grzegorz.swiderski@nordicsemi.no>