i2c: designware: introducing I2C_SLAVE definitions

- Definitions were added to core library
- A example was added to designware-core.txt Documentation that shows
  how the slave can be setup using DTS

SLAVE related definitions were added to the core of the controller.

Signed-off-by: Luis Oliveira <lolivei@synopsys.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>