[Core] Add Raspberry Pi RP2040 support (#14877)

* Disable RESET keycode because of naming conflicts

* Add Pico SDK as submodule

* Add RP2040 build support to QMK

* Adjust USB endpoint structs for RP2040

* Add RP2040 bootloader and double-tap reset routine

* Add generic and pro micro RP2040 boards

* Add RP2040 onekey keyboard

* Add WS2812 PIO DMA enabled driver and documentation

Supports regular and open-drain output configuration. RP2040 GPIOs are
sadly not 5V tolerant, so this is a bit use-less or needs extra hardware
or you take the risk to fry your hardware.

* Adjust SIO Driver for RP2040

* Adjust I2C Driver for RP2040

* Adjust SPI Driver for RP2040

* Add PIO serial driver and documentation

* Add general RP2040 documentation

* Apply suggestions from code review

Co-authored-by: Nick Brassel <nick@tzarc.org>

Co-authored-by: Nick Brassel <nick@tzarc.org>

1 files changed, 39 insertions, 0 deletions

diff --git a/docs/flashing.md b/docs/flashing.md
index dfb255f2dc..5e5dcb34e4 100644
--- a/docs/flashing.md
+++ b/docs/flashing.md
@@ -362,3 +362,42 @@ CLI Flashing sequence:
 ### `make` Targets
 
 * `:uf2-split-left` and `:uf2-split-right`: Flashes the firmware but also sets the handedness setting in EEPROM by generating a side specific firmware.
+
+## Raspberry Pi RP2040 UF2
+
+The `rules.mk` setting for this bootloader is `rp2040`, and can be specified at the keymap or user level.
+
+To ensure compatibility with the rp2040 bootloader, make sure this block is present in your `rules.mk`:
+
+```make
+# Bootloader selection
+BOOTLOADER = rp2040
+```
+
+Compatible flashers:
+
+* Any application able to copy a file from one place to another, such as _macOS Finder_ or _Windows Explorer_.
+
+Flashing sequence:
+
+1. Enter the bootloader using any of the following methods:
+    * Tap the `QK_BOOTLOADER` keycode
+    * Hold the `BOOTSEL` button on the PCB while plugin in the usb cable.
+    * Double-tap the `RESET` button on the PCB<sup>1</sup>.
+2. Wait for the OS to detect the device
+3. Copy the .uf2 file to the new USB disk
+4. Wait for the keyboard to become available
+
+or
+
+CLI Flashing sequence:
+
+1. Enter the bootloader using any of the following methods:
+    * Tap the `QK_BOOTLOADER` keycode
+    * Hold the `BOOTSEL` button on the PCB while plugin in the usb cable.
+    * Double-tap the `RESET` button on the PCB<sup>1</sup>.
+2. Wait for the OS to detect the device
+3. Flash via QMK CLI eg. `qmk flash --keyboard handwired/onekey/rpi_pico --keymap default`
+4. Wait for the keyboard to become available
+
+<sup>1</sup>: This works only if QMK was compiled with `RP2040_BOOTLOADER_DOUBLE_TAP_RESET` defined.