ppc: qemu: Delete the temporary FDT virtual-physical mapping after U-Boot is relocated

After U-Boot is relocated to RAM already, the previous temporary FDT
virtual-physical mapping that was used in the pre-relocation phase
is no longer needed. Let's delete the mapping.

get_fdt_virt() might be used before and after relocation, update it
to return different virtual address of FDT.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>