buildroot

support/testing: TestATFMarvell needs BR2_TARGET_UBOOT_NEEDS_DTC=y

The Marvell U-Boot needs DTC to build, so the test case fails to build
on systems where dtc isn't available. We add
BR2_TARGET_UBOOT_NEEDS_DTC=y to make sure that we build our own DTC.

Fixes:

  https://gitlab.com/buildroot.org/buildroot/-/jobs/44126707

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>