ARM: dts: at91: Add dts files for at91sam9260ek/9g20ek

The device tree source files of at91sam9g20ek and at91sam9260ek
boards are copied from the Linux v4.10, do the changes below.
 - Fix the build error for the usb0 node.
 - Add the reg property for the pinctrl node.
 - Move the gpio (pioA, pioB, pioC ...) nodes as the pinctrl's
   slibling nodes, instead of the child nodes.
 - Add the "u-boot,dm-pre-reloc" property for the dbgu node are used
   in board_init_f stage.
 - Add the clk pinctrl of the mmc0 node.
 - Fix the compilation warnings.

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