powerpc: Move 64-bit feature fixup earlier

Make it part of early_setup() as we really want the feature fixups
to be applied before we turn on the MMU since they can have an impact
on the various assembly path related to MMU management and interrupts.

This makes 64-bit match what 32-bit does.

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