We have a PCI-E host at toybrick board, which supports up to PCI-E gen2 with 4 lanes.

A sample of device tree

&pcie0 {
        ep-gpios = <&gpio0 12 GPIO_ACTIVE_HIGH>;
        num-lines = <4>;
        max-link-speed = <2>;
        pinctrl-names = "default";
        pinctrl-0 = <&pcie_clkreqn_cpm &pcie_ep>;
        status = "okay";
};

ep-gpios: reset pins for this device.

num-lines: the max lane would be used for this device, up to 4 lanes

max-link-speed: the max speed 1 for GEN1 and 2 for GEN2

Enabling the device drive(We use RTL8169 as example here)

Select the device driver at kernel configure menu(we use menuconfig here):

After the kernel is refreshed, we can the device from the output of lspci:

You would find a ethernet device is added to system as well: