am33xx changed BOOT_DEVICE_SPI to correct value

Changed BOOT_DEVICE Code for SPI on AM33xx.
According AM335x reference manual page 4960
(SPRUH73L-October 2011-Revised February 2015)
Reviewed-by: NLokesh Vutla <lokeshvutla@ti.com>

arch/arm/include/asm/arch-am33xx/spl.h

@@ -43,7 +43,7 @@
 #define BOOT_DEVICE_NAND_I2C	0x06
 #define BOOT_DEVICE_MMC1	0x08
 #define BOOT_DEVICE_MMC2	0x09
-#define BOOT_DEVICE_SPI		0x15
+#define BOOT_DEVICE_SPI		0x0B
 #define BOOT_DEVICE_UART	0x41
 #define BOOT_DEVICE_USBETH	0x44
 #define BOOT_DEVICE_CPGMAC	0x46