ARM: imx: fix: Provide correct enum values for ONENAND/NOR boot recognition

ARM: imx: fix: Provide correct enum values for ONENAND/NOR boot recognition

According to "Table 5-1. Boot Device Select" (page 335,
i.MX 6Dual/6Quad Applications Processor Reference Manual, Rev. 4, 09/2017)
the BOOT_CFG1[3] have following values (regarding EIM booting):
0 - NOR flash and 1 - ONENAND

This commit provides correct identification of the boot medium for IMX6Q
boards booting from NOR memory (MCCMON6 is one of them).

Signed-off-by: Lukasz Majewski <lukma@denx.de>