of: remove CONFIG_OF_DEVICE

CONFIG_OF_DEVICE is always selected when CONFIG_OF is enabled, so remove
it and simplify of_platform.h and of_device.h headers. This also fixes
!OF compiles using of_platform_populate.

Signed-off-by: Rob Herring <rob.herring@calxeda.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Tested-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Grant Likely <grant.likely@linaro.org>