tcl/fpga/xilinx-dna: Support for reading Spartan3 DNA code

Add Xilinx Spartan3 ISC_DNA instruction

Signed-off-by: George Voicu <razvanvg@hotmail.com>
Change-Id: Iaddb079c9fdd1b91c65def36878fe81783098696
Reviewed-on: https://review.openocd.org/c/openocd/+/7331
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>

tcl/fpga/xilinx-dna.cfg

@@ -1,7 +1,9 @@
 # SPDX-License-Identifier: GPL-2.0-or-later
 
+# Spartan3: Table 9-5 in https://www.xilinx.com/support/documentation/user_guides/ug332.pdf
 proc xilinx_dna_addr {chip} {
 	array set addrs {
+		Spartan3 0x31
 		Spartan6 0x30
 		Series7 0x17
 	}
@@ -43,3 +45,7 @@ proc xc7_get_dna {tap} {
 proc xc6s_get_dna {tap} {
 	return [xilinx_get_dna $tap Spartan6]
 }
+
+proc xc3s_get_dna {tap} {
+	return [xilinx_get_dna $tap Spartan3]
+}