soc_core: Add option to override CSR base

soc_core: Add option to override CSR base

When creating standalone IP cores such as standalone LiteDRAM without
a CPU, the CSR are presented externally via a wishbone with just enough
address bits to access individual CSRs (14), and no address decoding
otherwise. It is expected that the design using such core will have
its own address decoder gating cyc/stb.

However, such a design might still need to use LiteX code such as
the sdram init code, which relies on the generated csr.h. Thus we
want to be able to control the CSR base address used by that generated
csr.h.

This could be handled instead by having the "host" code provide
modified csr_{read,write}_simple() that include the necessary base
address. However, such an approach would make things complicated
if the design includes multiple such standalone cores with separate
CSR busses (such as LiteDRAM and LiteEth).

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