dt: bindings: move guts devicetree doc out of powerpc directory

Move guts devicetree doc to Documentation/devicetree/bindings/soc/fsl/
since it's used by not only PowerPC but also ARM. And add a specification
for 'little-endian' property.

Signed-off-by: Yangbo Lu <yangbo.lu@nxp.com>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Scott Wood <oss@buserror.net>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>

Documentation/devicetree/bindings/soc/fsl/guts.txt

@@ -25,6 +25,9 @@ Recommended properties:
  - fsl,liodn-bits : Indicates the number of defined bits in the LIODN
    registers, for those SOCs that have a PAMU device.
 
+ - little-endian : Indicates that the global utilities block is little
+   endian. The default is big endian.
+
 Examples:
 	global-utilities@e0000 {	/* global utilities block */
 		compatible = "fsl,mpc8548-guts";