libfdt: Add functions for handling the "compatible" property

[Jon Loeliger]

So, like, the other day David Gibson mumbled:
> This patch adds functions for dealing with the compatible property.
> fdt_node_check_compatible() can be used to determine whether a node is
> compatible with a given string and fdt_node_offset_by_compatible()
> locates nodes with a given compatible string.
> 
> Testcases for these functions are also included.
> 
> Signed-off-by: David Gibson <david at gibson.dropbear.id.au>

Applied.

jdl