fdt: Add function to get config int from device tree

Add a function to look up a configuration item such as machine id
and return its value.

Note: The code has been taken as is from the Chromium u-boot development
tree and needs Simon Glass' sign-off.

Signed-off-by: Abhilash Kesavan <a.kesavan@samsung.com>
Signed-off-by: Simon Glass <sjg@chromium.org>