PCI: dra7xx: Simplify probe code with devm_gpiod_get_optional()

No functional change.  Use the new devm_gpiod_get_optional() to simplify
the probe code.

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