i2c: at91: document clock properties

Document the clock properties required by the at91 i2c bus driver.

Signed-off-by: Boris BREZILLON <b.brezillon@overkiz.com>
Acked-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>