Add support to use second and third I2C bus, too.

Bus 0 is still the default, but by calling i2c_set_bus_num(1/2) before doing
I2C accesses, code can switch to bus 1 and 2, too. Don't forget to switch
back afterwards, then.

Signed-off-by: Dirk Behme <dirk.behme@googlemail.com>