fu740_pci_dw: Fix PERST delay and keep asserted for rest of reset sequence

DELAY takes microseconds not milliseconds, so 100 was too low. Moreover,
when enabling hw.pci.clear_pcib, PCI emeration would still stop at one
of the first bridges, but by asserting PERST for the rest of the reset
sequence that appears to be reliably addressed.

Fixes: 896e217a0eae ("fu740_pci_dw: Add SiFive FU740 PCIe controller driver")