fdt: fix out of tree builds with DT support

Fix: FATAL ERROR: Couldn't open "../arch/arm/dts/tegra20.dtsi": No
such file or directory

Signed-off-by: Wolfgang Denk <wd@denx.de>
Cc: Simon Glass <sjg@chromium.org>
Acked-by: Simon Glass <sjg@chromium.org>

dts/Makefile

@@ -36,7 +36,7 @@ $(error Your architecture does not have device tree support enabled. \
 Please define CONFIG_ARCH_DEVICE_TREE))
 
 # We preprocess the device tree file provide a useful define
-DTS_CPPFLAGS := -DARCH_CPU_DTS=\"../arch/$(ARCH)/dts/$(CONFIG_ARCH_DEVICE_TREE).dtsi\"
+DTS_CPPFLAGS := -DARCH_CPU_DTS=\"$(SRCTREE)/arch/$(ARCH)/dts/$(CONFIG_ARCH_DEVICE_TREE).dtsi\"
 
 all:	$(obj).depend $(LIB)