phy/model: Don't generate empty mem_*.init files

When using the SDRAM PHY model without specified init data, the
generator still generates a bunch of $readmemh for each bank
reading mem.init, mem_1.init etc... all of which are 0-sized files.

This is cumbersome especially when using a standalone model in
an external project.

This is fixed by having the default bank_init be set to a list
of "None" rather than a list of empty lists.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

litedram/phy/model.py

@@ -492,7 +492,7 @@ class SDRAMPHYModel(Module):
             self.submodules += timing_checker
 
         # Bank init data ---------------------------------------------------------------------------
-        bank_init  = [[] for i in range(nbanks)]
+        bank_init  = [None for i in range(nbanks)]
 
         if init:
             bank_init = self.__prepare_bank_init_data(