boards: Add Banana Pi BPI-F3 development board documentation

Signed-off-by: Ariel D'Alessandro <ariel.dalessandro@collabora.com>

content/boards/spacemit-k1-bananapi-f3.md

+---
+title: Banana Pi BPI-F3 (SpacemiT K1)
+---
+
+The [Banana Pi BPI-F3](https://docs.banana-pi.org/en/BPI-F3/BananaPi_BPI-F3)
+is an industrial grade RISC-V development board, designed with
+[SpacemiT K1](https://docs.banana-pi.org/en/BPI-F3/SpacemiT_K1) 8 core RISC-V
+chip.
+
+Specifications:
+
+* CPU: SpacemiT K1 Octa-core X60 (RV64GCVB)
+* GPU: IMG BXE-2-32 @ 819MHz
+* Memory: LPDDR4 (up to 16G)
+* Storage: 8/16/32/128 GB eMMC flash (optional 4M SPI NOR, 32M SPI NAND)
+  * SD card slot
+* Wireless: 2.4G/5G WiFi and Bluetooth 4.2
+* Ethernet: 2x GbE Ethernet port
+* HDMI: 1x HDMI 1.4 output, up to 1080p@60fps
+* Display: MIPI DSI interface
+* Audio: Speaker, Mic, Earphone
+* Camera: MIPI-CSI, Dual-shot support
+* USB: 4x USB 3.0 Type-A HOST, 1x USB 2.0 Type-C OTG
+* PCIe:
+  * 2x PCIE2.1 2lane
+  * 1x PCIEB 2lane connect M.2 KEY-M
+  * 1x PCIEC 1lane connect MINI PCIE
+* GPIO: 26-pin header
+* Buttons: Reset, Power and Burn
+* Power: DC in and TYPE-C in
+
+### Power supply
+
+The board is powered with a DC12V/3A supply via the DC power jack. It can also
+be powered via USB Type-C with 12V/3A.
+
+### Serial console
+
+The board has a serial (115200 8N1) console connection available on the
+`UART0 Debug` connector.
+
+![Banana Pi BPI-F3 connectors](/img/spacemit-k1-bananapi-f3.jpg)
+
+### FDL button - download key
+
+For remote bootmode control, the FDL button (`SW2` - download key) can have
+soldered wires. Note that this isn't just a GND pulldown, but a pull up when
+pressed.
+
+![Banana Pi BPI-F3 FDL button](/img/spacemit-k1-bananapi-f3-fdl-button.jpg)
+
+### Bootloader
+
+The Banana Pi BPI-F3 still lacks of upstream u-boot with ethernet support. In
+order to enable tftp-booting, the vendor u-boot bootchain is used. Artifacts
+can be downloaded from:
+
+https://gitlab.collabora.com/lava/spacemit-k1-bananapi-f3-boot/
+
+#### Flashing bootloader
+
+For reference on the bootloader flashing process and binaries, see
+https://gitlab.collabora.com/lava/spacemit-k1-bananapi-f3-boot/#flash-firmware