imx: Use IMX6_BMODE_* macros instead of numericals

Use meaningful macros IMX6_BMODE_*, instead of numerical
number in boot mode detection code.

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