pci: pcie-brcmstb: bring over some robustness improvements from Linux

Since the initial U-Boot driver was ported here from Linux, the latter
has had a few changes for robustness/stability. This patch brings over
two of them:
- Do not attempt to access the configuration space of a PCIe device if
  the link has gone down, as that will result in an asynchronous SError
  interrupt which will crash U-Boot.
- Wait for the recommended 100ms after PERST# is deasserted.

I sent this patch while debugging a crash involving PCIe, but these
are unrelated improvements. I do not believe that this patch fixes any
real-world bug.

Signed-off-by: Sam Edwards <CFSworks@gmail.com>