pci: Make Rockchip PCIe voltage regulators optional

pci: Make Rockchip PCIe voltage regulators optional

The vpcie*-supply properties are optional and these are absent on
boards like the ROCKPro64 and Firefly RK3399 where the voltage is
supplied by always-on regulators that are already enabled upon
boot.  Make these regulators optional and properly check their
presence before attempting to enable them.

Makes PCIe work on un U-Boot on the boards mentioned above.

Signed-off-by: Mark Kettenis <kettenis@openbsd.org>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>
Tested-by: Marcin Juszkiewicz <marcin@juszkiewicz.com.pl>