arm64: dts: rockchip: add variables for pcie completion to helios64

arm64: dts: rockchip: add variables for pcie completion to helios64

without ep-gpios defined u-boot does not initialise PCIe
rockchip_pcie pcie@f8000000: failed to find ep-gpios property

additionally set max-link-speed and pinctrl-names for completeness

with this patch and the ones from Florian Klink applied to the dts
file in u-boot sata drives show up in both u-boot and linux

Signed-off-by: Dennis Gilmore <dgilmore@redhat.com>
Acked-By: Florian Klink <flokli@flokli.de>
Link: https://lore.kernel.org/r/20211029005323.144652-1-dgilmore@redhat.com
Signed-off-by: Heiko Stuebner <heiko@sntech.de>