dts/Makefile: Turn off some predefined macros

Add '-ansi' to DTS_CPPFLAGS to avoid unwanted expansion of dts content
that matches some predefined macros.

Example: A number of PowerPC related *.dts files in the kernel define a
property named 'linux,network-index' which (w/o '-ansi') is expanded to
'1,network-index' by the preprocessor because of '#define linux 1.'
Signed-off-by: NHorst Kronstorfer <hkronsto@frequentis.com>

dts/Makefile

@@ -36,7 +36,8 @@ $(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=\"$(SRCTREE)/arch/$(ARCH)/dts/$(CONFIG_ARCH_DEVICE_TREE).dtsi\" \
+DTS_CPPFLAGS := -ansi \
+		-DARCH_CPU_DTS=\"$(SRCTREE)/arch/$(ARCH)/dts/$(CONFIG_ARCH_DEVICE_TREE).dtsi\" \
 		-DBOARD_DTS=\"$(SRCTREE)/board/$(VENDOR)/$(BOARD)/dts/$(DEVICE_TREE).dts\"
 
 all:	$(obj).depend $(LIB)