riscv: dts: sifive: fu540-c000: align dma node name with dtschema

Fixes dtbs_check warnings like:

  dma@3000000: $nodename:0: 'dma@3000000' does not match '^dma-controller(@.*)?$'

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Link: https://lore.kernel.org/r/20220407193856.18223-1-krzysztof.kozlowski@linaro.org
Fixes: c5ab54e9

 ("riscv: dts: add support for PDMA device of HiFive Unleashed Rev A00")
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>