powerpc: Create pci_controller_ops.window_alignment and shim

Add pci_controller_ops.window_alignment,
shadowing ppc_md.pcibios_window_alignment.
Add a shim, and changes the callsites to use the shim.

Here, we use pci_window_alignment, as pcibios_window_alignment is
already taken.

Signed-off-by: Daniel Axtens <dja@axtens.net>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>