PCI: designware: Require config accesses to be naturally aligned

Add sanity checks on "addr" input parameter in dw_pcie_cfg_read() and
dw_pcie_cfg_write().  These checks make sure that accesses are aligned on
their size, e.g., a 4-byte config access is aligned on a 4-byte boundary.

[bhelgaas: changelog, set *val = 0 in failure case]
Signed-off-by: Gabriele Paoloni <gabriele.paoloni@huawei.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Pratyush Anand <pratyush.anand@gmail.com>