ppc: qemu: Drop a custom env variable 'fdt_addr_r'

Now that we have switched to CONFIG_OF_CONTROL, and we can use the
env variable 'fdtcontroladdr' directly instead of creating one that
is duplicated.

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