powerpc/85xx/ge_imp3a: Don't use the flat device-tree after boot

ge_imp3a_pic_init() is called way beyond the unflattening of
the tree, it shouldn't be using of_flat_dt_*

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>