riscv: dts: starfive: replace underscores in node names

Underscores should not be used in node names (dtc with W=2 warns about
them), so replace them with hyphens.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>