riscv: dts: Add hifive-unleashed-a00 dts from Linux

Sync the hifive-unleashed-a00 dts from Linux with
below commit details:

commit <2993c9b04e616df0848b655d7202a707a70fc876> ("riscv: dts: HiFive
Unleashed: add default chosen/stdout-path")

Idea is to periodically sync the dts from Linux instead of
tweaking internal changes one after another, so better not
add any intermediate changes in between. This would help to
maintain the dts files easy and meaningful since we are
reusing device tree files from Linux.

Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Anup Patel <anup.patel@wdc.com>