ARM: at91: enhance linux-at91 at91sam9x5 support

This patch enhances the linux-at91 at91sam9x5 support mainly by updating
device tree files to use latest drivers and to enable dma.
at91_dt_defconfig has also been updated to support features available on
at91sam9x5 family.

Known issue:
- dma-rx for dbgu is not working
- no support for audio
- no support for ISI
- no support for dataflash memory on Cogent cpu modules

Signed-off-by: Ludovic Desroches <ludovic.desroches@atmel.com>
dt: define of_find_device_by_node when dt is not enabled

Temporary workaround since some at91 drivers use this function even if dt is
not enabled causing compilation issue.

Signed-off-by: Ludovic Desroches <ludovic.desroches@atmel.com>