PCI: armada: Remove redundant struct armada8k_pcie.base

The struct armada8k_pcie.base pointer is always a constant offset from
struct pcie_port.dbi_base.  Encode that offset in the register macros so we
don't need to maintain the armada8k_pcie.base pointer.  No functional
change intended.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>