fdt: fix dts preprocessor options

Using "-ansi" preprocessor option will cause dts lines that begin with
'#' to choke the preprocessor.  Change to "-x assembler-with-cpp"
instead which is what the kernel uses to preprocess dts files.

Signed-off-by: Allen Martin <amartin@nvidia.com>
Reviewed-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Tom Warren <twarren@nvidia.com>