mmc: atmel-mci: add pdc support and runtime capabilities detection

Add pdc support for atmel-mci. It makes at91-mci driver useless because it
was only used for the old atmel MCI core which has pdc but no dma support.
To allow removing at91-mci, the capabilities of the MCI core are detected
at runtime -- then the driver will use pio, pdc or dma transfers.
Warning: at91rm9200 is not supported, to support it we need to use swab32
on data but I have no board to test it.

Signed-off-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Chris Ball <cjb@laptop.org>