Revert "Microblaze: build kernel with device tree"

This is way too specific to microblaze-only. Remove this support to
introduce a more generic way to do support device tree kernels.

This reverts commit aaed42d1.

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