doc

Device Tree really improves code maintainability and is now
available for SPL too.

This is the state-of-the-art implementation in U-boot.

The board files (platform data) are no longer needed.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Marek Vasut <marex@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>