libfdt: Add fdt_set_name() function

[Jon Loeliger]

So, like, the other day David Gibson mumbled:
> This patch adds an fdt_set_name() function to libfdt, mirroring
> fdt_get_name().  This is a r/w function which alters the name of a
> given device tree node.
> 
> Signed-off-by: David Gibson <david at gibson.dropbear.id.au>

Applied.

jdl