PCI: brcmstb: Split post-link up initialization to brcm_pcie_start_link()

PCI: brcmstb: Split post-link up initialization to brcm_pcie_start_link()

Previously brcm_pcie_setup() initialized the Root Port itself as well as
doing the actual link-up.  Split brcm_pcie_setup() into two functions:

  - brcm_pcie_setup(), which initializes everything that does not require
    the link itself to be up, and

  - brcm_pcie_start_link(), which brings up the link and initializes things
    that depend on the link being up.

[bhelgaas: condense commit log, deferring details for future changes]
Link: https://lore.kernel.org/r/20220725151258.42574-3-jim2101024@gmail.com
Signed-off-by: Jim Quinlan <jim2101024@gmail.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Tested-by: Florian Fainelli <f.fainelli@gmail.com>