nitrogen6x: Populate FDTFILE at build-time for all platforms

Rather than using CONFIG_SABRELITE to set FDTFILE for only that
platform, switch to always setting this based on
CONFIG_DEFAULT_DEVICE_TREE as this should always match the kernel
device tree name anyhow.

Cc: Troy Kisky <troy.kisky@boundarydevices.com>
Signed-off-by: Tom Rini <trini@konsulko.com>
Acked-by: Troy Kisky <troy.kisky@boundarydevices.com>