fdt_support: Add helper function to read "ranges" property

This patch adds a helper function that can be used to interpret most
"ranges" properties in the device tree.

It reads the n'th range out of a "ranges" array and returns the node's
virtual address of the range, the physical address that range starts at
and the size of the range.

Signed-off-by: Alexander Graf <agraf@suse.de>
Acked-by: Scott Wood <scottwood@freescale.com>
Reviewed-by: York Sun <yorksun@freescale.com>