buildroot

configs/{at91sam9x5ek*, atmel_*_xplained*}: U-Boot needs DTC

Since the bump of the Atmel defconfigs to linux4sam_5.6 in commit
0f7b5d4a9f3a78be07e22c81db41f88e45e8c0cf, the new U-Boot version uses
the Device Tree compiler, so we need to add BR2_TARGET_UBOOT_NEEDS_DTC
in order to build dtc before u-boot and avoid build failures.

Fixes:

>>> uboot linux4sam_5.6 Configuring
>>> uboot linux4sam_5.6 Building
[...]
  CHK     include/generated/timestamp_autogenerated.h
  UPD     include/generated/timestamp_autogenerated.h
./scripts/dtc-version.sh: line 17: dtc: command not found
./scripts/dtc-version.sh: line 18: dtc: command not found
*** Your dtc is too old, please upgrade to dtc 1.4 or newer

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>