soc_core: add new alloc_mem/add_mem_region to allow automatic allocation of memory...

soc_core: add new alloc_mem/add_mem_region to allow automatic allocation of memory regions

With add_memory_region, user needs to provide the memory origin, which should not be needed since
could be retrieved from mem_map and prevent automatic allocation which is already possible for csr
and interrupts.

New add_mem_region method now allows both: defining the memory origin in mem_map (which will then
be used) or let the SoC builder automatically find and allocate a memory region.