nitrogen6x: Remove extra quotes in fdtfile

nitrogen6x: Remove extra quotes in fdtfile

After commit 395110284b56 ("nitrogen6x: Populate FDTFILE at build-time
for all platforms") iMX.6Q Sabrelite fails to boot due to a bad fdtfile
string:

  Retrieving file: /dtbs/5.18.0-0.deb11.4-armmp/"imx6q-sabrelite".dtb
  ** File not found /dtbs/5.18.0-0.deb11.4-armmp/"imx6q-sabrelite".dtb **

CONFIG_DEFAULT_DEVICE_TREE option is string typed, so __stringify() is
adding extra quotes. Remove this.

Signed-off-by: Ariel D'Alessandro <ariel.dalessandro@collabora.com>
Reviewed-by: Fabio Estevam <festevam@denx.de>
Acked-by: Troy Kisky <troy.kisky@boundarydevices.com>