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: Horst Kronstorfer <hkronsto@frequentis.com>