i2c: at91: update documentation for DT bindings

add a new value "atmel,sama5d2-i2c" for the "compatible" property.
add a new optional property "atmel,fifo-size" to enable FIFO support when
available.
add missing optional properties "dmas" and "dma-names".

Signed-off-by: Cyrille Pitchen <cyrille.pitchen@atmel.com>
Acked-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>