of: search scripts/dtc/include-prefixes path for both CPP and DTC

Since commit d5d332d3 ("devicetree: Move include prefixes from
arch to separate directory"), cross-arch DT reference works well,
but only for CPP style #include directives.

It makes as much sense to share DT between different architectures
by using DTC's /include/ directives.

So, scripts/dtc/include-prefixes should be passed to both CPP and DTC.
I refactored Makefile.lib a bit to not repeat the same path.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Rob Herring <robh@kernel.org>