soc_core: use fixed 16MB CSR address space

Using too small CSR address space cause a regression on PCIe SoC, this would
need to be understood if we want to reduce CSR address space under 16MB.

litex/soc/integration/soc_core.py

@@ -298,7 +298,8 @@ class SoCCore(Module):
         self.add_csr_master(self.wishbone2csr.csr)
         self.config["CSR_DATA_WIDTH"] = csr_data_width
         self.config["CSR_ALIGNMENT"] = csr_alignment
-        self.register_mem("csr", self.soc_mem_map["csr"], self.wishbone2csr.wishbone, 2**(csr_address_width + 2))
+        assert 2**(csr_address_width + 2) <= 0x1000000
+        self.register_mem("csr", self.soc_mem_map["csr"], self.wishbone2csr.wishbone, 0x1000000)
 
         # Add UART
         if with_uart: