[dcd] Rename reg top to u_reg

Using the same instance name is easier to support CSR backdoor access
In other IPs, we all use u_reg

Signed-off-by: Weicai Yang <weicai@google.com>
diff --git a/hw/ip/dcd/rtl/dcd.sv b/hw/ip/dcd/rtl/dcd.sv
index c2e905b..05911a7 100644
--- a/hw/ip/dcd/rtl/dcd.sv
+++ b/hw/ip/dcd/rtl/dcd.sv
@@ -47,7 +47,7 @@
   logic nc_intg_err_o;//FIXME
 
   // Register module
-  dcd_reg_top i_reg_top (
+  dcd_reg_top u_reg (
     .clk_i(clk_i),
     .rst_ni(rst_ni),
     .tl_i(tl_i),