powerpc: Merge 32-bit and 64-bit setup_arch()

There is little enough differences now.

mpe: Add a/p/k/setup.h to contain the prototypes and empty versions of
functions we need, rather than using weak functions. Add a few other
empty versions to avoid as many #ifdefs as possible in the code.

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