of/fdt: fix argument name and add comments of unflatten_dt_node()

Match the name of the third argument in the comment block
to the actual function: p -> poffset

Add missing comments about the arguments "nodepp" and "dryrun".

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Grant Likely <grant.likely@linaro.org>