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>