xilinx.dcm: restore description of -$CMP XC2S64A-XQFG48

Also change it ('X' -> 'x') to match the one in xilinx.lib.
The description was accidentally removed in
784c023be1.

library/xilinx.dcm

@@ -4,6 +4,11 @@ $CMP XC18V01SO20
 F xilinx/xc18v00proms.pdf
 $ENDCMP
 #
+$CMP XC2S64A-xQFG48
+D Xilinx CoolRunner
+F xilinx/xc2c64.pdf
+$ENDCMP
+#
 $CMP XC2S100TQ144
 D spartan 2
 K FPGA