hw/riscv: virt: Convert fdt_load_addr to uint64_t

hw/riscv: virt: Convert fdt_load_addr to uint64_t

fdt_load_addr was previously declared as uint32_t which doe not match
with the return type of riscv_compute_fdt_addr().

This patch modifies the fdt_load_addr type from a uint32_t to a uint64_t
to match the riscv_compute_fdt_addr() return type.

This fixes calculating the fdt address when DRAM is mapped to higher
64-bit address.

Reviewed-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Signed-off-by: Lakshmi Bai Raja Subramanian <lakshmi.bai.rajasubramanian@bodhicomputing.com>
[ Change by AF:
 - Cleanup commit title and message
]
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <168872495192.6334.3845988291412774261-1@git.sr.ht>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>