rp2040: Add initial support for the rp2040 mcu

Support the rp2040 (as tested on a Raspberry Pi Pico board).  This
adds basic uart, timer, gpio, and watchdog support.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

src/rp2040/Kconfig

@@ -0,0 +1,59 @@
+# Kconfig settings for STM32 processors
+
+if MACH_RP2040
+
+config RP2040_SELECT
+    bool
+    default y
+    select HAVE_GPIO
+    select HAVE_GPIO_BITBANGING
+    select HAVE_STRICT_TIMING
+
+config BOARD_DIRECTORY
+    string
+    default "rp2040"
+
+config MCU
+    string
+    default "rp2040"
+
+config CLOCK_FREQ
+    int
+    default 12000000
+
+config FLASH_SIZE
+    hex
+    default 0x200000
+
+config RAM_START
+    hex
+    default 0x20000000
+
+config RAM_SIZE
+    hex
+    default 0x42000
+
+config STACK_SIZE
+    int
+    default 512
+
+config FLASH_START
+    hex
+    default 0x10000000
+
+######################################################################
+# Communication inteface
+######################################################################
+
+config USBSERIAL
+    bool
+config SERIAL
+    bool
+choice
+    prompt "Communication interface"
+    config RP2040_SERIAL_UART0
+        bool "Serial (on UART0 GPIO1/GPIO0)"
+        select SERIAL
+endchoice
+
+endif