common/board_f: make sure to call fix_fdt() before reserve_fdt()

There may be a chance that board specific fix_fdt() will change the
size of FDT blob so it's safe to call reserve_fdt() after fix_fdt()
otherwise global data (gd) will overwrite with FDT blob values.

Fixes: a8492e25 ("riscv: Expand the DT size before copy reserved memory node")

Signed-off-by: Pragnesh Patel <pragnesh.patel@sifive.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Rick Chen <rick@andestech.com>
Reviewed-by: Atish Patra <atish.patra@wdc.com>