PCI: rockchip: Make 'ep-gpios' DT property optional

The Rockchip PCIe controller DT binding clearly states that 'ep-gpios' is
an optional property. And indeed there are boards that don't require it.

Make the driver follow the binding by using devm_gpiod_get_optional()
instead of devm_gpiod_get().

[bhelgaas: tidy whitespace]
Link: https://lore.kernel.org/r/20210121162321.4538-2-wens@kernel.org
Fixes: e77f847d ("PCI: rockchip: Add Rockchip PCIe controller support")
Fixes: 956cd99b ("PCI: rockchip: Separate common code from RC driver")
Fixes: 964bac94

 ("PCI: rockchip: Split out rockchip_pcie_parse_dt() to parse DT")
Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>