rockchip: add support for Pine64 RockPro64

rockchip: add support for Pine64 RockPro64

This adds the new rockchip target and support for RockPro64 RK3399

Flash:    16 MiB SPI NOR
RAM:      2 GiB/4 GiB LPDDR4
SoC:      RK3399
USB:      2x USB 2.0, 1x USB 3.0, 1x USB-C
Ethernet: 1x GbE
PCIe:     PCIe 2.0, 4 lanes
Storage:  eMMC or SD card
Optional SDIO wifi/bt module

The Pine64 RockPro64 is a single-board-computer with a 4x PCIe connector,
6 ARM64 cores (4 little, 2 big), plenty of RAM and storage.

By default the single Gigabit-Ethernet port is configured as the
LAN port.

Installation of the firware is possible by dd'ing the image
to an SD card or the eMMC flash.

Serial: 3v3 1500000 8n1

U-boot is build from the mainline tree and
integrated into the images. Required ATF to build u-boot
is downloaded from a CI build bot.

Signed-off-by: Tobias Mädel <t.maedel@alfeld.de>
Tested-by: Tobias Schramm <t.schramm@manjaro.org>