fsl/mpic: Document and use the "big-endian" device-tree flag

The MPIC code checks for a "big-endian" property and sets the flag
MPIC_BIG_ENDIAN if one is present, although prior to the "mpic->flags"
fixup that would never have worked anways.

Unfortunately, even now that it works properly, the Freescale mpic
device-node (the "PowerQUICC-III"-compatible one) does not specify it,
so all of the board ports need to manually pass it to mpic_alloc().

Document the flag and add it to the pq3 device tree.  Existing code will
still need to pass the MPIC_BIG_ENDIAN flag because their dtb may not
have this property, but new platforms shouldn't need to do so.
Signed-off-by: Kyle Moffett <Kyle.D.Moffett@boeing.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

Documentation/devicetree/bindings/powerpc/fsl/mpic.txt

@@ -56,7 +56,14 @@ PROPERTIES
           to the client.  The presence of this property also mandates
           that any initialization related to interrupt sources shall
           be limited to sources explicitly referenced in the device tree.
-       
+
+  - big-endian
+      Usage: optional
+      Value type: <empty>
+          If present the MPIC will be assumed to be big-endian.  Some
+          device-trees omit this property on MPIC nodes even when the MPIC is
+          in fact big-endian, so certain boards override this property.
+
 INTERRUPT SPECIFIER DEFINITION
 
   Interrupt specifiers consists of 4 cells encoded as

arch/powerpc/boot/dts/fsl/pq3-mpic.dtsi

@@ -39,6 +39,7 @@ mpic: pic@40000 {
 	reg = <0x40000 0x40000>;
 	compatible = "fsl,mpic";
 	device_type = "open-pic";
+	big-endian;
 };
 
 timer@41100 {