board: sama5d3_xplained: Update to support DM/DT

Update the configuration files to support the device tree and driver
model, so do SPL. The device clock and pins configuration are handled
by the clock and the pinctrl drivers respectively.

Signed-off-by: Wenyou Yang <wenyou.yang@atmel.com>
Fix build error with sama5d3_xplained_mmc:
Signed-off-by: Simon Glass <sjg@chromium.org>