platform: sifive/fu540: Move FDT further up

This patch changes FW_xyz_FDT_ADDR to RAM_START+128MB (i.e. 0x88000000)
so that next stage bootloader (i.e. U-Boot) has enough space to unpack
bigger kernel images.

Reported-by: David Abdurachmanov <david.abdurachmanov@sifive.com>
Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: David Abdurachmanov <david.abdurachmanov@sifive.com>
Tested-by: David Abdurachmanov <david.abdurachmanov@sifive.com>