riscv: dts: add initial board data for the SiFive HiFive Unleashed

Add initial board data for the SiFive HiFive Unleashed A00.

Currently the data populated in this DT file describes the board
DRAM configuration and the external clock sources that supply the
PRCI.

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: Rob Herring <robh+dt@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Palmer Dabbelt <palmer@sifive.com>
Cc: Albert Ou <aou@eecs.berkeley.edu>
Cc: Antony Pavlov <antonynpavlov@gmail.com>
Cc: devicetree@vger.kernel.org
Cc: linux-riscv@lists.infradead.org
Cc: linux-kernel@vger.kernel.org