of: remove #ifdef from linux/of_platform.h

A lot of code uses the functions from of_platform.h when built for
devicetree-enabled platforms but can also be built without them.
In order to avoid using #ifdef everywhere in drivers, this
makes all the function declarations visible, which means we
can use "if (IS_ENABLED(CONFIG_OF))" in driver code and get build
coverage over the code but let the compiler drop the reference
in the object code.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Grant Likely <grant.likely@linaro.org>
Cc: Rob Herring <rob.herring@calxeda.com>
Signed-off-by: Grant Likely <grant.likely@linaro.org>