Move the installation of DMA operations out of swiotlb's subsys
initcall, and into the generic PCI controller operations struct.

These ops are installed conditionally, based on the ppc_swiotlb_enable
global. The global can be set in two places:
 - swiotlb_detect_4g, which is always called at the arch initcall level
 - setup_pci_atmu, which is called as part of the fsl_add_bridge and
fsl_pci_syscore_do_resume.

fsl_pci_syscore_do_resume is called late enough that any changes as a
result of that call will have no effect.

As such, if we test the global and set the operations as part of
fsl_add_bridge, after the call to setup_pci_atmu, we can be confident
that it will cover all the PCI implementations affected by the changes
to dma-swiotlb.c.
Signed-off-by: Daniel Axtens <dja@axtens.net>
Acked-by: Scott Wood <scottwood@freescale.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>