Add driver for Rockchip PCIe root complex found in RK3399 SOC.
Unfortunately, there are some limitations:

• memory aperture of his controller is only 16MiB, so it is nearly unusable for graphic cards
• every attempt to generate type 1 config cycle always causes trap. These config cycles are disabled now and we don't support cards with PCIe switch.
• in some cases, attempt to do config cycle to (probably) not-yet ready card also causes trap. This cannot be detected at runtime, but it seems like very rare issue.

MFC after: 3 weeks
Differential Revision: https://reviews.freebsd.org/D22724