i2c: add i2c_core and prepare for new multibus support

This Patch introduce the new i2c_core file, which holds
the I2C core functions, for the rework of the multibus/
multiadapter support.
Also adds changes in i2c.h for the new I2C multibus/multiadapter
support. This new support can be activated with the
CONFIG_SYS_I2C define.

Signed-off-by: Heiko Schocher <hs@denx.de>
Signed-off-by: Simon Glass <sjg@chromium.org>
Cc: Mike Frysinger <vapier@gentoo.org>
Cc: Stephen Warren <swarren@wwwdotorg.org>
diff --git a/drivers/i2c/Makefile b/drivers/i2c/Makefile
index 72e85a3..06b211d 100644
--- a/drivers/i2c/Makefile
+++ b/drivers/i2c/Makefile
@@ -46,6 +46,7 @@
 COBJS-$(CONFIG_U8500_I2C) += u8500_i2c.o
 COBJS-$(CONFIG_SH_I2C) += sh_i2c.o
 COBJS-$(CONFIG_SH_SH7734_I2C) += sh_sh7734_i2c.o
+COBJS-$(CONFIG_SYS_I2C) += i2c_core.o
 COBJS-$(CONFIG_ZYNQ_I2C) += zynq_i2c.o
 
 COBJS	:= $(COBJS-y)