fdt: Add a function to count strings

Given a device tree node and a property name, the fdt_stringlist_count()
function counts the number of strings found in the property value.

This also adds a new error code, FDT_ERR_BADVALUE, that the function
returns when it encounters a non-NUL-terminated string list.

Signed-off-by: Thierry Reding <treding@nvidia.com>
[Changed testcase name --dwg]
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>