integration/builder: fix building for SoCSDRAM-based targets when SDRAM is disabled

Reported by Florent Kermarrec

misoc/integration/builder.py

@@ -53,7 +53,7 @@ class Builder:
         flash_boot_address = getattr(self.soc, "flash_boot_address", None)
         csr_regions = self.soc.get_csr_regions()
         constants = self.soc.get_constants()
-        if isinstance(self.soc, soc_sdram.SoCSDRAM):
+        if isinstance(self.soc, soc_sdram.SoCSDRAM) and self.soc._sdram_phy:
             sdram_phy_settings = self.soc._sdram_phy[0].settings
         else:
             sdram_phy_settings = None