riscv: dts: hifive-unleashed-a00: Make memory node available to SPL

Make memory node available to SPL in prepration to updates to SiFive
DDR RAM driver to read memory information from DT.

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