PCI: iproc: Convert to use the new ECAM constants

PCI: iproc: Convert to use the new ECAM constants

Change interface of the function iproc_pcie_map_ep_cfg_reg() so that use
of PCI_SLOT() and PCI_FUNC() macros and most of the local ECAM-specific
constants can be dropped, and the new PCIE_ECAM_OFFSET() macro can be
used instead.  Use the ALIGN_DOWN() macro to ensure that PCI Express
ECAM offset is always 32 bit aligned.

Suggested-by: Bjorn Helgaas <bhelgaas@google.com>
Link: https://lore.kernel.org/r/20201129230743.3006978-4-kw@linux.com
Signed-off-by: Krzysztof Wilczyński <kw@linux.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>