cmd_eeprom: make it possible to define the used i2c bus

A SoC like the i.MX6 supports more then one i2c bus. In oder to be
able to use the eeprom command add a new define to specify the
i2c bus to use. If CONFIG_SYS_I2C_EEPROM_BUS is not defined there
is no functional change, else a call to i2c_set_bus_num(..) is
done before calling i2c_read(..) and i2c_write(..).

Signed-off-by: Christian Gmeiner <christian.gmeiner@gmail.com>
Acked-by: Stefano Babic <sbabic@denx.de>