[POWERPC] i2c: Fix build breakage introduced by OF helpers

Fix build breakage introduced in commit "[POWERPC] i2c: OF helpers for
the i2c API".  If i2c-core is compiled as a module, the helper needs
to be compiled as a module, as well.  Rename i2c.c to of_i2c.c to
avoid name space conflict.

[paulus@samba.org: Changed dependency from OF to PPC_OF to avoid
sparc{32,64} allmodconfig breakage.]
Signed-off-by: Jochen Friedrich <jochen@scram.de>
Signed-off-by: Paul Mackerras <paulus@samba.org>

drivers/of/Kconfig

@@ -9,7 +9,7 @@ config OF_GPIO
 	  OpenFirmware GPIO accessors
 
 config OF_I2C
-	def_bool y
-	depends on OF && I2C
+	def_tristate I2C
+	depends on PPC_OF && I2C
 	help
 	  OpenFirmware I2C accessors

drivers/of/Makefile

 obj-y = base.o
 obj-$(CONFIG_OF_DEVICE) += device.o platform.o
 obj-$(CONFIG_OF_GPIO)   += gpio.o
-obj-$(CONFIG_OF_I2C)	+= i2c.o
+obj-$(CONFIG_OF_I2C)	+= of_i2c.o