imx: spl: Update NAND bootmode detection bit

BOOT_CFG1[7:4] the NAND boot mode selection is done
only when BOOT_CFG1[7] is 1 hence update the NAND
boot mode detection bit case. This information available
on Table 8-11. NAND Boot eFUSE Descriptions, from IMX6DQRM.

Cc: Tim Harvey <tharvey@gateworks.com>
Reviewed by: Stefano Babic <sbabic@denx.de>
Signed-off-by: Jagan Teki <jagan@openedev.com>

arch/arm/imx-common/spl.c

@@ -63,8 +63,8 @@ u32 spl_boot_device(void)
 	case 0x6:
 	case 0x7:
 		return BOOT_DEVICE_MMC1;
-	/* NAND Flash: 8.5.2 */
-	case 0x8 ... 0xf:
+	/* NAND Flash: 8.5.2, Table 8-10 */
+	case 0x8:
 		return BOOT_DEVICE_NAND;
 	}
 	return BOOT_DEVICE_NONE;