<linux/of_platform.h>: fix compilation warnings with DT disabled

Fix the following compilation warnings (in Simon Horman's renesas.git repo):

In file included from arch/arm/mach-shmobile/setup-r8a7779.c:24:0:
include/linux/of_platform.h:107:13: warning: ‘struct of_device_id’ declared
inside parameter list [enabled by default]
include/linux/of_platform.h:107:13: warning: its scope is only this definition
or declaration, which is probably not what you want [enabled by default]
include/linux/of_platform.h:107:13: warning: ‘struct device_node’ declared
inside parameter list [enabled by default]

<linux/of_platform.h> only #include's headers with definitions of the above
mentioned structures if CONFIG_OF_DEVICE=y but uses them even if not. One
solution is to move some #include's out of #ifdef CONFIG_OF_DEVICE and use
incomplete declarations for the rest of the structures where the #ifdef move
doesn't help...

Reported-by: Vladimir Barinov <vladimir.barinov@cogentembedded.com>
Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Rob Herring <rob.herring@calxeda.com>