i2c: at91: add dma support

Add dma support for Atmel TWI which is available on sam9x5 and later.

When using dma for reception, you have to read only n-2 bytes. The last
two bytes are read manually. Don't doing this should cause to send the
STOP command too late and then to get extra data in the receive
register.
Signed-off-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>