integration/soc: review/simplify changes for standalone cores.

integration/soc: review/simplify changes for standalone cores.

- do the CSR alignment update only if CPU is not CPUNone.
- revert PointToPoint interconnect when 1 master and 1 slave since this will
break others use cases and will prevent mapping slave to a specific location.
It's probably better to let the synthesis tools optimize the 1:1 mapping directly.
- add with_soc_interconnect parameter to add_sdram that defaults to True. When
set to False, only the LiteDRAMCore will be instantiated and interconnect with
the SoC will not be added.