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: NPaul Walmsley <paul.walmsley@sifive.com>
Signed-off-by: NPaul Walmsley <paul@pwsan.com>
Tested-by: NLoys Ollivier <lollivier@baylibre.com>
Tested-by: NKevin 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