boards: Add rockpi4 documentation

Signed-off-by: Nícolas F. R. A. Prado <nfraprado@collabora.com>

content/boards/rockpi4.md

+
+---
+title: ROCK Pi 4
+---
+
+Note: A lot of the information here was taken from [Collabora's RockPi4
+repository][2].
+
+ROCK Pi 4 is a Rockchip RK3399 based SBC (Single Board Computer) by Radxa.
+
+There are three variants: A, B and C. With the main difference being the
+presence of WiFi and Bluetooth on B and C, and the presence of DP and HDMI mini
+on C. More information on https://wiki.radxa.com/Rockpi4/getting_started.
+
+Specification:
+* Rockchip Rk3399 SoC
+* 2x ARM Cortex-A72 @ 1.8GHz, 4x ARM Cortex-A53 @ 1.4GHz
+* 1/2/4GB memory
+* Mali T860MP4 GPU
+* MIPI CSI 2 lanes connector
+* eMMC module connector
+* uSD slot (up to 128GB)
+* 4x USB
+* 1x HDMI
+* Ethernet port
+* 40-pin IO header including UART, SPI, I2C and 5V DC power in
+* USB PD over USB Type-C
+* Size: 85mm x 54mm
+
+This board is available from [Radxa distributors](https://wiki.radxa.com/Buy).
+
+### Power control
+
+The board can be powered on with 5V via pins 2 or 4 in the GPIO header (as shown
+in the [GPIO pinout][1]), and GND on pin 6 for example. The pins are colored
+accordingly on the latest board revision:
+
+![Power through GPIO](/img/rockpi4-power-gpio.jpg)
+
+Alternatively, the board can be powered via a USB PD Type-C connector:
+* 9V/2A, 12V/2A, 15V/2A, 20V/2A
+
+### Low-level boot control
+
+Serial connection on the board can be done through pins on the GPIO. The board's
+TX is on pin 8 and RX on pin 10 as per [the pinout][1]. Serial specification:
+* 3.3V TTL
+* 1500000 8n1
+
+![Serial through GPIO](/img/rockpi4-serial-gpio.jpg)
+
+#### Network connectivity
+
+There's a standard Gigabit Ethernet port on the board that can be used.
+
+### Bootloader
+
+For the bootloader setup, a ready-to-use image is available at [Collabora's Rock
+Pi 4 repository][2] as an artifact from the CI. In addition to u-boot (and
+idbloader and trust img), it also contains a kernel and rootfs, but after
+flashing and dropping to the u-boot prompt, tftp and nfs can be configured to
+retrieve the kernel and rootfs, ignoring the ones flashed.
+
+Load addresses:
+* Kernel: 0x280000
+* DTB: 0x8300000
+* Ramdisk: 0xa200000
+
+The main bootloader is stored in eMMC.
+
+### Health checks
+
+Mainline kernel and defconfig don't have working NFS. For that, [this custom
+config](https://people.collabora.com/~nfraprado/config-rockpi4-v5_12) can be
+used with mainline v5.12 kernel.
+
+### Lab notes and trouble shooting
+
+None.
+
+[1]: https://wiki.radxa.com/Rockpi4/hardware/gpio
+[2]: https://gitlab.collabora.com/rockpi/rockpi4