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>
diff --git a/include/configs/nitrogen6x.h b/include/configs/nitrogen6x.h
index 2007b48..26e6de2 100644
--- a/include/configs/nitrogen6x.h
+++ b/include/configs/nitrogen6x.h
@@ -76,7 +76,7 @@
 	"fdt_high=0xffffffff\0" \
 	"initrd_high=0xffffffff\0" \
 	"fdt_addr_r=0x18000000\0" \
-	"fdtfile=" __stringify(CONFIG_DEFAULT_DEVICE_TREE) ".dtb\0" \
+	"fdtfile=" CONFIG_DEFAULT_DEVICE_TREE ".dtb\0" \
 	"kernel_addr_r=" __stringify(CONFIG_SYS_LOAD_ADDR) "\0"  \
 	"pxefile_addr_r=" __stringify(CONFIG_SYS_LOAD_ADDR) "\0" \
 	"scriptaddr=" __stringify(CONFIG_SYS_LOAD_ADDR) "\0" \