ARM: dts: i.MX25: define SSI FIFO depth

According to the i.MX25 reference manuals, each SSI has four FIFOs. All
of those FIFOs can store up to 15 entries.

The fsl_ssi driver's internal default for the FIFO depth in 8. Set our
non-default FIFO depth explicitly in the Device Tree.
Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

arch/arm/boot/dts/imx25.dtsi

@@ -269,6 +269,7 @@ ssi2: ssi@50014000 {
 				dmas = <&sdma 24 1 0>,
 				       <&sdma 25 1 0>;
 				dma-names = "rx", "tx";
+				fsl,fifo-depth = <15>;
 				status = "disabled";
 			};
 
@@ -329,6 +330,7 @@ ssi1: ssi@50034000 {
 				dmas = <&sdma 28 1 0>,
 				       <&sdma 29 1 0>;
 				dma-names = "rx", "tx";
+				fsl,fifo-depth = <15>;
 				status = "disabled";
 			};