utils: Use cpu_to_fdt32() when writing to fdt

Although it does not make a functional difference, both cpu_to_fdt32()
and fdt32_to_cpu() pass on to CPU_TO_FDT32, we should use cpu_to_fdt32()
to be semantically correct.

Signed-off-by: Georg Kotheimer <georg.kotheimer@kernkonzept.com>
Reviewed-by: Atish Patra <atish.patra@wdc.com>
Acked-by: Anup Patel <anup.patel@wdc.com>