board: at91sam9261ek: Update to support DT and DM

Add the dts files to support deivce tree, update the configuration
files to support the device tree and driver model. The peripheral
clock and pins configuration are handled by the clock and the pinctrl
drivers respectively.

Enable the early debug UART to debug problems when an ICE or other
debug mechanism is not available.

Signed-off-by: Wenyou Yang <wenyou.yang@microchip.com>
Reviewed-by: Simon Glass <sjg@chromium.org>