spi: Update NPCM PSPI controller documentation

Update the PSPI NPCM binding document of the spi aliases use
to define the spi ID number.

Signed-off-by: Tomer Maimon <tmaimon77@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/spi/nuvoton,npcm-pspi.txt

@@ -19,8 +19,16 @@ Optional properties:
 - clock-frequency : Input clock frequency to the PSPI block in Hz.
 		    Default is 25000000 Hz.
 
+Aliases:
+- All the SPI controller nodes should be represented in the aliases node using
+  the following format 'spi{n}' withe the correct numbered in "aliases" node.
+
 Example:
 
+aliases {
+	spi0 = &spi0;
+};
+
 spi0: spi@f0200000 {
 	compatible = "nuvoton,npcm750-pspi";
 	reg = <0xf0200000 0x1000>;