[POWERPC] Cleanup mpic nodes in .dts

Removed clock-frequency, big-endian, and built-in props as they aren't
specified anywhere.  Also added compatible = "chrp,open-pic" in the
places it was missing.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>

arch/powerpc/boot/dts/sbc8548.dts

@@ -265,12 +265,10 @@
 		mpic: pic@40000 {
 			interrupt-controller;
 			#address-cells = <0>;
-			#size-cells = <0>;
 			#interrupt-cells = <2>;
 			reg = <0x40000 0x40000>;
 			compatible = "chrp,open-pic";
 			device_type = "open-pic";
-                        big-endian;
 		};
 	};