ARM: configs: at91: DT/ATAG defconfig modifications

As all AT91 platforms are converted to DT for a long time, adapt the
defconfigs by:
- removing legacy CONFIG_ATAGS as a DT will always be provided;
- removing the CONFIG_ARM_APPENDED_DTB option on SAMA5 devices as the
  vast majority of systems will use a DT-aware bootloader on these
  devices.

Signed-off-by: Nicolas Ferre <nicolas.ferre@microchip.com>