ARM: dts: at91: add dts files for sama5d4 Xplained

ARM: dts: at91: add dts files for sama5d4 Xplained

Add the device tree files for sama5d4 Xplained board.

The dts files are copied from Linux-4.4, do the following changes.
 - add reg property for pinctrl node.
 - move the gpio nodes(pioA, pioB, pioC ...) from the pinctrl child's
   nodes to its slibling nodes.
 - add the "u-boot,dm-pre-reloc" property to determine which nodes
   which are needed by SPL and by the board_init_f stage.
 - fix the compilation warnings.

Signed-off-by: Wenyou Yang <wenyou.yang@atmel.com>