uboot-imx

dts/Makefile: don't use cpp -P

Recent dtc supports #line directives in the input source code, and even
uses them to generate useful line numbers in any messages it emits. Stop
passing -P to cpp, since there's no need any more.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Simon Glass <sjg@chromium.org>