xtensa: standardize devicetree cpu compatible strings

The recommended compatible string format, according to the ePAPR v1.1 standard,
is "manufacturer,model". Change the xtensa cpu compatible strings to
"cdns,xtensa-cpu". Also, change the boards compatible strings in a similar way.

The pic compatible string will be dealt with in a separate patch.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>

arch/xtensa/boot/dts/lx60.dts

@@ -3,7 +3,7 @@
 /include/ "xtfpga-flash-4m.dtsi"
 
 / {
-	compatible = "xtensa,lx60";
+	compatible = "cdns,xtensa-lx60";
 	memory@0 {
 		device_type = "memory";
 		reg = <0x00000000 0x04000000>;