machine: Add board support for BigTreeTech SKR Pico (#4842)

* machine: add support for BTT SKR Pico
Adds support for the BigTreeTech SKR Pico 3D-printer mainboard.
This board uses the RP2040.
* Fix build tag
* Add I2C defaults
* Run UART test instead of blinky1
* Use NoPin for I2C and SPI on BTT SKR Pico
* Cleanup comments
* Don't use ADC pin names

targets/btt-skr-pico.json

@@ -0,0 +1,13 @@
+{
+    "inherits": [
+        "rp2040"
+    ],
+    "build-tags": ["btt_skr_pico"],
+    "serial-port": ["2e8a:000A"],
+    "ldflags": [
+        "--defsym=__flash_size=16M"
+    ],
+    "extra-files": [
+        "targets/pico-boot-stage2.S"
+    ]
+}