board: at91sam9263ek: Use SPI-flash-based AT45xxx DataFlash

To support driver model and device tree, use the SPI-flash-based
AT45xxx DataFlash driver, DataFlash is a kind of SPI flash.
Instead of ATMEL_DATAFLASH_SPI DataFlash older driver that will
be removed in the future.

Signed-off-by: Wenyou Yang <wenyou.yang@microchip.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/arm/dts/at91sam9263ek.dts b/arch/arm/dts/at91sam9263ek.dts
index 8cd7fad..35799b8 100644
--- a/arch/arm/dts/at91sam9263ek.dts
+++ b/arch/arm/dts/at91sam9263ek.dts
@@ -87,7 +87,7 @@
 				cs-gpios = <&pioA 5 0>, <0>, <0>, <0>;
 				mtd_dataflash@0 {
 					compatible = "atmel,at45", "atmel,dataflash";
-					spi-max-frequency = <50000000>;
+					spi-max-frequency = <15000000>;
 					reg = <0>;
 				};
 			};