i2c: add i2c_core and prepare for new multibus support

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 <[email protected]>
Signed-off-by: Simon Glass <[email protected]>
Cc: Mike Frysinger <[email protected]>
Cc: Stephen Warren <[email protected]>