arch: riscv: add support for building DTB files from DT source data

Similar to ARM64, add support for building DTB files from DT source
data for RISC-V boards.

This patch starts with the infrastructure needed for SiFive boards.
Boards from other vendors would add support here in a similar form.
Signed-off-by: Paul Walmsley <paul.walmsley@sifive.com>
Signed-off-by: Paul Walmsley <paul@pwsan.com>
Tested-by: Loys Ollivier <lollivier@baylibre.com>
Tested-by: Kevin Hilman <khilman@baylibre.com>
Cc: Palmer Dabbelt <palmer@sifive.com>
Cc: Albert Ou <aou@eecs.berkeley.edu>

arch/riscv/boot/dts/Makefile

+# SPDX-License-Identifier: GPL-2.0
+subdir-y += sifive