powerpc: Move 64-bit memory reserves to setup_arch()

There is really no need to do them that early, early_setup() runs
before MMU is on, we should do the strict minimum there to get the
MMU going.

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