core: use wider DFI address/bank if PHY requires it

[jedrzejboczar]

This is a part of changes related to #224.

LiteDRAM normally encodes Mode Register Set command using DFIInjector, passing mode register address as dfi.baddress and value as dfi.address. In LPDDR4 there are more Mode Registers than in other DRAMs and we need 6 bits to store the address. So when a module has only 8 banks it is not possible to issue MRS via DFIInjector this way.

Initially I tried encoding both the address and the value in dfi.address, but then I had to modify the implementation of sdram_write_leveling_on/sdram_write_leveling_off in sdram.c for LPDDR4 specifically. To avoid it, now LPDDR4 PHY sets bankbits to 6: dfd7816#diff-d84b2ae2991ef50d505ebd78f702957557f81bdfcd77f1aafbdd7bc66cc86ae5L97, and with this PR DFIInjector would use the higher value of geom_settings.bankbits and phy.bankbits, making sure that there are enough bits for Mode Register address.

[enjoy-digital]

Thanks @jedrzejboczar, this is indeed a nice way to avoid extra changes in the other part of the code.