[nrf fromtree] samples: subsys: usb: mass: Include rpi_pico.overlay

Include rpi_pico.overlay from the rpi_pico_rp2040_w.overlay in order to
have `pico/rp2040/w` board to work out of the box.

Signed-off-by: John Lin <[email protected]>
(cherry picked from commit 0fe7cbd)

Author: jlbwoods

samples/subsys/usb/mass/boards/rpi_pico_rp2040_w.overlay

@@ -0,0 +1 @@
+#include "rpi_pico.overlay"