riscv: dts: HiFive Unleashed: add default chosen/stdout-path

Add a default "stdout-path" to the kernel DTS file, as is present in many
of the board DTS files elsewhere in the kernel tree. With this line
present, earlyconsole can be enabled by simply passing "earlycon" on the
kernel command line.  No specific device details are necessary, since the
kernel will use the stdout-path as the default.

Signed-off-by: Paul Walmsley <paul.walmsley@sifive.com>
Reviewed-by: Atish Patra <atish.patra@wdc.com>