[otbn,rtl] Delete redundant register in rf_base

We deal with call stack (x1) in a different module (otbn_stack)
so there is no need to instantiate x1 inside rf_base.

Signed-off-by: Canberk Topal <ctopal@lowrisc.org>

hw/ip/otbn/rtl/otbn_rf_base_ff.sv

diff --git a/hw/ip/otbn/rtl/otbn_rf_base_ff.sv b/hw/ip/otbn/rtl/otbn_rf_base_ff.sv
index 41d16db..3aac3f1 100644
--- a/hw/ip/otbn/rtl/otbn_rf_base_ff.sv
+++ b/hw/ip/otbn/rtl/otbn_rf_base_ff.sv
@@ -44,8 +44,11 @@
   // No flops for register 0 as it's hard-wired to 0
   assign rf_reg[0] = WordZeroVal;
 
+  // No flops for register 1 as it's call stack and handled in a different module.
+  assign rf_reg[1] = WordZeroVal;
+
   // Generate flops for register 1 - NGpr
-  for (genvar i = 1; i < NGpr; i++) begin : g_rf_flops
+  for (genvar i = 2; i < NGpr; i++) begin : g_rf_flops
     logic [BaseIntgWidth-1:0] rf_reg_q;
 
     always_ff @(posedge clk_i) begin