hw/riscv: Support to load DTB after 3GB memory on 64-bit system.

hw/riscv: Support to load DTB after 3GB memory on 64-bit system.

Larger initrd image will overlap the DTB at 3GB address. Since 64-bit
system doesn't have 32-bit addressable issue, we just load DTB to the end
of dram in 64-bit system.

Signed-off-by: Jim Shu <jim.shu@sifive.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Message-ID: <20241120153935.24706-2-jim.shu@sifive.com>
[ Changes by AF
 -  Store fdt_load_addr_hi32 in the reset vector
]
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>