libfdt: Add function to explicitly expand aliases

[Jon Loeliger]

> Kumar has already added alias expansion to fdt_path_offset().
> However, in some circumstances it may be convenient for the user of
> libfdt to explicitly get the string expansion of an alias.  This patch
> adds a function to do this, fdt_get_alias(), and uses it to implement
> fdt_path_offset().
> 
> Signed-off-by: David Gibson <david at gibson.dropbear.id.au>

Applied.

jdl