AT91SAM9260EK: Normalize SPI timings

This patch changes the SPI timings to closely match the ones
used by the Linux kernel and the Atmel's own bootstrap project.

Signed-off-by: Stelian Pop <stelian@popies.net>
Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>

include/configs/at91sam9260ek.h

@@ -96,7 +96,7 @@
 #define CFG_MAX_DATAFLASH_BANKS		2
 #define CFG_DATAFLASH_LOGIC_ADDR_CS0	0xC0000000	/* CS0 */
 #define CFG_DATAFLASH_LOGIC_ADDR_CS1	0xD0000000	/* CS1 */
-#define AT91_SPI_CLK			33000000
+#define AT91_SPI_CLK			15000000
 #define DATAFLASH_TCSS			(0x1a << 16)
 #define DATAFLASH_TCHS			(0x1 << 24)