OF: Add a warning in case chosen node is not present

On architectures that only get their bootargs through devicetree's
chosen node (such as RISC-V), that node is mandatory. After a
discussion with Rob [1] I'm adding a warning in case chosen node
is not present, to let users know about it.

[1]: https://patchwork.ozlabs.org/patch/984224/#2016136



Signed-off-by: Nick Kossifidis <mick@ics.forth.gr>
Reviewed-by: Palmer Dabbelt <palmer@sifive.com>
Signed-off-by: Rob Herring <robh@kernel.org>