of: Document exactly what of_find_node_by_name() puts

It isn't clear if this function of_node_put()s the 'from'
argument, or the node it searches. Clearly indicate which
variable is touched. Fold in some more fixes from Randy too
because we're in the area.

Cc: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Rob Herring <robh@kernel.org>