of: define CONFIG_OF globally so architectures can select it

Signed-off-by: NStephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: NGrant Likely <grant.likely@secretlab.ca>

drivers/of/Kconfig

+config OF
+	bool
+
 config OF_FLATTREE
 	bool
 	depends on OF