fdt: fix dts preprocessor options

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 <[email protected]>
Reviewed-by: Stephen Warren <[email protected]>
Acked-by: Simon Glass <[email protected]>