i2c: break out smbus support into separate file

Break out the exported SMBus functions and the emulation layer into a
separate file. This also involved splitting up the tracing header into
an I2C and an SMBus part.

Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>

Documentation/driver-api/i2c.rst

@@ -44,3 +44,6 @@ i2c_adapter devices which don't support those I2C operations.
 
 .. kernel-doc:: drivers/i2c/i2c-core-base.c
    :export:
+
+.. kernel-doc:: drivers/i2c/i2c-core-smbus.c
+   :export: