rp2040: add chipid support

The rp2040 doesn't have a chip ID, but the flash chip connected does. We
can get this ID by asking the flash chip directly, but doing so requires
disengaging the XIP layer, performing the interrogation of the flash
chip, and then re-enabling the XIP layer. This gives us a 64-bit unique
ID that we can use as our USB serial number.

Signed-off-by: Lasse Dalegaard <dalegaard@gmail.com>

src/rp2040/Kconfig

@@ -9,6 +9,7 @@ config RP2040_SELECT
     select HAVE_GPIO_ADC
     select HAVE_GPIO_BITBANGING
     select HAVE_STRICT_TIMING
+    select HAVE_CHIPID
 
 config BOARD_DIRECTORY
     string