dts/Makefile: unify cpp/dtc include paths

*.dts may use #include (via cpp) or /include/ (via dtc; assuming a newer
dtc). The choice is up to the creator of the DT. Create a common variable
DTC_INCDIRS that lists the paths searched by include statements, and
update cpp and dtc invocation to use them.

For cpp, also specify -nostdinc to ensure the same set of paths is
available to both type of include statement.

For dtc, create a new DTC_FLAGS variable to hold all the flags passed to
dtc.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Simon Glass <sjg@chromium.org>

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