imx: spl: Fix NAND bootmode detection

commit 20f14714 ("imx: spl: Update NAND bootmode detection bit")
broke the NAND bootmode detection by checking if
BOOT_CFG1[7:4] == 0x8 for NAND boot mode.
This commit essentially reverts it, while using the IMX6_BMODE_*
macros that were introduced since.

Tables 8-7 & 8-10 from IMX6DQRM say the NAND boot mode selection
is done when BOOT_CFG1[7] is 1, but BOOT_CFG1[6:4] is not
necessarily 0x0 in this case.
Actually, NAND boot mode is when 0x8 <= BOOT_CFG1[7:4] <= 0xf,
like it was in the code before.

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