[doc] Update section on DIFs in hw/design.md

Signed-off-by: Amaury Pouly <amaury.pouly@lowrisc.org>

Author: pamaury

doc/contributing/hw/design.md

@@ -125,22 +125,12 @@ Generating both top and cmdgen can be done conveniently with
 $ make -C hw top_and_cmdgen
 ```
 
-### New DIF
+### Update DIFs
 
 Unfortunately, re-generating TOP does not resolve everything.
-If the interrupt names are changed or new interrupts are introduced, the DIF for the IP block should be re-generated.
-
-```console
-$ ./util/make_new_dif.py --mode=regen --only=autogen
-```
-
-The command above updates the DIF (auto-generated part) under `sw/device/lib/dif/`.
-
-If DIF for the IP block already uses the interrupt enums inside, you need to manually revise the reference.
-In most cases, the interrupt name is `kDif`, followed by the IP name in PascalCase (e.g `SpiDevice`), then `Irq`, then the interrupt name in PascalCase (e.g. `RxFull`).
-For example, `kDifSpiDeviceIrqRxFull`.
-To refer to an interrupt in the top-level PLIC enum, the format is `kTopEarlgreyPlicIrqId`, followed by the IP name in PascalCase, then the interrupt name in PascalCase.
-For example, `kTopEarlgreyPlicIrqIdSpiDeviceRxFull`.
+If the interrupt names are changed or new interrupts are introduced, the DIF for the IP block should be updated.
+The DIFs are split between an automatically generated part and a manually written one.
+The autogenerated code will automatically pick up changes but the manual part may need to be revised manually.
 
 ### PLIC Unittest