am335x_evm: Correct CONFIG_CMD_SPL_WRITE_SIZE

We use CONFIG_CMD_SPL_WRITE_SIZE when reading/writing the args portion
of falcon mode to NAND.  Previously it was half the size of the
eraseblock which is too small, increase to eraseblock size.
Signed-off-by: NTom Rini <trini@ti.com>

include/configs/am335x_evm.h

@@ -344,7 +344,7 @@
 /* nand */
 #define CONFIG_CMD_SPL_NAND_OFS			0x240000 /* end of u-boot */
 #define CONFIG_SYS_NAND_SPL_KERNEL_OFFS		0x280000
-#define CONFIG_CMD_SPL_WRITE_SIZE		0x1000
+#define CONFIG_CMD_SPL_WRITE_SIZE		0x2000
 
 /* spl export command */
 #define CONFIG_CMD_SPL