ex1: Add SPR_TBU support

It's used by the boot wrapper in Linux and possibly some userspace
programs.
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

common.vhdl

@@ -27,6 +27,7 @@ package common is
     constant SPR_DSISR  : spr_num_t := 18;
     constant SPR_DAR    : spr_num_t := 19;
     constant SPR_TB     : spr_num_t := 268;
+    constant SPR_TBU    : spr_num_t := 269;
     constant SPR_DEC    : spr_num_t := 22;
     constant SPR_SRR0   : spr_num_t := 26;
     constant SPR_SRR1   : spr_num_t := 27;

execute1.vhdl

@@ -752,6 +752,9 @@ begin
 		    case decode_spr_num(e_in.insn) is
 		    when SPR_TB =>
 			spr_val := ctrl.tb;
+		    when SPR_TBU =>
+                        spr_val(63 downto 32) := (others => '0');
+			spr_val(31 downto 0)  := ctrl.tb(63 downto 32);
 		    when SPR_DEC =>
 			spr_val := ctrl.dec;
                     when 724 =>     -- LOG_ADDR SPR