dt-bindings: riscv: fix the schema compatible string for the HiFive Unleashed board

dt-bindings: riscv: fix the schema compatible string for the HiFive Unleashed board

[ Upstream commit b390e0bfd2996f1215231395f4e25a4c011eeaf9 ]

The YAML binding document for SiFive boards has an incorrect
compatible string for the HiFive Unleashed board.  Change it to match
the name of the board on the SiFive web site:

   https://www.sifive.com/boards/hifive-unleashed

which also matches the contents of the board DT data file:

   https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/riscv/boot/dts/sifive/hifive-unleashed-a00.dts#n13

Signed-off-by: Paul Walmsley <paul.walmsley@sifive.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>