[POWERPC] fsl_spi: stop using device_type = "spi"

Also:
- rename "fsl_spi" to "fsl,spi";
- add and use cell-index property, if found;
- split probing code out of fsl_spi_init, thus we can call
  it for legacy device_type probing and new "compatible" probing.

Signed-off-by: Anton Vorontsov <avorontsov@ru.mvista.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>

arch/powerpc/boot/dts/mpc8349emitxgp.dts

@@ -80,8 +80,8 @@
 		};
 
 		spi@7000 {
-			device_type = "spi";
-			compatible = "fsl_spi";
+			cell-index = <0>;
+			compatible = "fsl,spi";
 			reg = <7000 1000>;
 			interrupts = <10 8>;
 			interrupt-parent = < &ipic >;