PCI: rcar-gen4: Assure reset occurs before DBI access

Assure the reset is latched and the core is ready for DBI access. On R-Car
V4H, the PCIe reset is asynchronous and does not take effect immediately,
but needs a short time to complete. In case DBI access happens in that
short time, that access generates an SError. Make sure that condition can
never happen, read back the state of the reset, which should turn the
asynchronous reset into a synchronous one, and wait a little over 1ms to
add additional safety margin.

Fixes: 0d0c5510 ("PCI: rcar-gen4: Add R-Car Gen4 PCIe controller support for host mode")
Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>
Signed-off-by: Manivannan Sadhasivam <mani@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Tested-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://patch.msgid.link/20250924005610.96484-1-marek.vasut+renesas@mailbox.org