hw/riscv/virt: Use setprop_sized_cells for rtc

hw/riscv/virt: Use setprop_sized_cells for rtc

The current device tree property uses two cells for the address (and for
the size), but assumes the they are less than 32 bits by hard coding the
high cell to zero.

Use qemu_fdt_setprop_sized_cells to do the job of splitting the upper
and lower 32 bits across cells.

Reviewed-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Joel Stanley <joel@jms.id.au>
Message-ID: <20250604025450.85327-11-joel@jms.id.au>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>