scripts/Makefile.dts: tweak logic for deciding which dtbs to build

The idea in 3609e1dc (dts: automatically build necessary .dtb
files) was fine, but the implementation was suboptimal due to some
misunderstandings on my part (and possibly defects in some defconfig
files):

- Sometimes DEFAULT_DEVICE_TREE is not included in OF_LIST or
  SPL_OF_LIST

- SPL_OF_LIST is not always a subset of OF_LIST

- While SPL_OF_LIST governs the list of dtbs relevant to SPL (i.e.,
  may be built into an
  SPL-with-bunch-of-dtbs-to-choose-between-at-runtime), those dtbs are
  not actually _built_ during the SPL build phase, i.e. when $(SPL_)
  would expand to SPL_. fdtgrep runs on the artifacts produced during
  the ordinary U-Boot build.

Tweak the logic so that we simply add the union of all dtbs mentioned
in either DEFAULT_DEVICE_TREE, OF_LIST and SPL_OF_LIST to dtb-y. That
should, for real, ensure that we always build all the dtbs that is
relevant to the current board, and should in turn enable us to
massively simplify arch/*/dts/Makefile.

Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Tested-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>