fdt_shrink_to_minimum: do not mark fdt space reserved unconditionally

This function merely relocates the fdt blob, so don't let it alter
it by adding reservations that didn't exist before.

Instead, if the memory used for the fdt blob has been reserved
before calling this function, ensure the relocated memory is
marked as reserved instead.

Reported-by: Keerthy <j-keerthy@ti.com>
Reported-by: Lokesh Vutla <lokeshvutla@ti.com>
Signed-off-by: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>
Reviewed-by: Lokesh Vutla <lokeshvutla@ti.com>