dt-bindings: riscv: d requires f

Per the specifications, the d extension for double-precision floating
point operations depends on the f extension for single-precision floating
point. Add that requirement to the bindings. This differs from the
Linux implementation, where single-precious only is not supported.

Reviewed-by: Clément Léger <cleger@rivosinc.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20250312-perpetual-daunting-ad489c9a857a@spud


Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>