am33xx: Add SPI SPL as an option

Add the required config.mk logic for this SoC as well as the BOOT_DEVICE
define.  Finally, enable the options on the am335x_evm.

Signed-off-by: Tom Rini <trini@ti.com>
diff --git a/arch/arm/include/asm/arch-am33xx/spl.h b/arch/arm/include/asm/arch-am33xx/spl.h
index 63ed10b..644ff35 100644
--- a/arch/arm/include/asm/arch-am33xx/spl.h
+++ b/arch/arm/include/asm/arch-am33xx/spl.h
@@ -27,6 +27,7 @@
 #define BOOT_DEVICE_NAND	5
 #define BOOT_DEVICE_MMC1	8
 #define BOOT_DEVICE_MMC2	9	/* eMMC or daughter card */
+#define BOOT_DEVICE_SPI		11
 #define BOOT_DEVICE_UART	65
 #define BOOT_DEVICE_CPGMAC	70
 #define BOOT_DEVICE_MMC2_2      0xFF