Add USE_CUSTOM_DFFRAM guard

- enable it to use the small custom DFFRAM; otherwise, use the generic
  RAM verilog model
- also updated the aspect ratio of the custom DFFRAM
diff --git a/verilog/rtl/caravel.v b/verilog/rtl/caravel.v
index d7c123f..23fb906 100644
--- a/verilog/rtl/caravel.v
+++ b/verilog/rtl/caravel.v
@@ -42,6 +42,7 @@
 `include "clock_div.v"
 `include "simple_por.v"
 `include "storage_bridge_wb.v"
+`include "DFFRAM.v"
 `include "sram_1rw1r_32_256_8_sky130.v"
 `include "storage.v"