PCI: dra7xx: Add support to make GPIO drive PERST# line

The PERST# line in am57x-evm is connected to a GPIO line and PERST# should
be driven high to indicate the clocks are stable (As per Figure 2-10: Power
Up of the PCIe CEM spec 3.0).

Add support to make GPIO drive PERST# line.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>