[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>
diff --git a/drivers/of/Makefile b/drivers/of/Makefile
index a07b953..548772e 100644
--- a/drivers/of/Makefile
+++ b/drivers/of/Makefile
@@ -1,4 +1,4 @@
 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