riscv: dts: hifive-unleashed-a00: add cpu aliases

Add cpu aliases to U-Boot specific dtsi for hifive-unleashed.
Without aliases we see that the CPU device sequence numbers are set
randomly and the cpu list/detail command will show it as follows:
=> cpu list
  1: cpu@1      rv64imafdc
  2: cpu@2      rv64imafdc
  3: cpu@3      rv64imafdc
  0: cpu@4      rv64imafdc

Seems like CPU probing with dm-model also relies on aliases as observed
in case spi. The fu540-c000-u-boot.dtsi has cpu nodes and so adding
corresponding aliases we can ensure that cpu devices are assigned
proper sequence as follows:

=> cpu list
  1: cpu@1      rv64imafdc
  2: cpu@2      rv64imafdc
  3: cpu@3      rv64imafdc
  4: cpu@4      rv64imafdc

Signed-off-by: Sagar Shrikant Kadam <sagar.kadam@sifive.com>
Reviewed-by: Pragnesh Patel <pragnesh.patel@sifive.com>
Reviewed-by: Bin Meng <bin.meng@windriver.com>