i2c: at91: added slave mode support

Slave mode driver is based on the concept of i2c-designware driver.

Signed-off-by: Juergen Fitschen <me@jue.yt>
[ludovic.desroches@microchip.com: rework Kconfig and replace IS_ENABLED
by defined]
Signed-off-by: Ludovic Desroches <ludovic.desroches@microchip.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>