Add missing sifive,fu540 compatible to fix dtbs_check warnings:

  arch/riscv/boot/dts/sifive/hifive-unleashed-a00.dt.yaml: /: compatible: 'oneOf' conditional failed, one must be fixed:
  ['sifive,hifive-unleashed-a00', 'sifive,fu540-c000'] is too short
  'sifive,hifive-unleashed-a00' is not one of ['sifive,hifive-unmatched-a00']
  'sifive,fu740-c000' was expected
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Reviewed-by: Alexandre Ghiti <alexandre.ghiti@canonical.com>
Tested-by: Alexandre Ghiti <alexandre.ghiti@canonical.com>
Link: https://lore.kernel.org/r/20210920130248.145058-2-krzysztof.kozlowski@canonical.com