fdt: Add a function to get the index of a string

The new fdt_stringlist_search() function will look up a given string in
the list contained in the value of a named property of a given device
tree node and return its index.

Signed-off-by: Thierry Reding <treding@nvidia.com>
[Fix some -Wshadow warnings --dwg]
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>