of: move definition of of_chosen into common code.

Rather than defining of_chosen in each arch, it can be defined for all
in driver/of/base.c

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Acked-by: Michal Simek <monstr@monstr.eu>