sof-dev-config: add CONFIG_USB_SERIAL_CP210X=m

As no one ever wanted to boot from a microphone, it's very rare that we
have to use the serial console to debug very early boot issues. But it
can happen, see for instance
thesofproject/linux#4799

In such case it's very convenient to use another test device to collect
logs on the serial console.

Signed-off-by: Marc Herbert <marc.herbert@intel.com>

sof-dev-defconfig

@@ -30,3 +30,15 @@ CONFIG_SND_SOC_SOF_SKYLAKE_SUPPORT=y
 
 # generic ACPI audio compositor
 CONFIG_SND_ACPI_AUDIO_COMPOSITOR=m
+
+# This USB-serial converter is embedded on current Intel development
+# boards.  Note the converter chip is on the USB gadget / Type-B / test
+# device side, whereas this cp210x.ko driver runs on the USB host /
+# Type-A / "workstation" side. So why include this driver on test
+# devices = on the "wrong" side?  Because it's much easier to connect
+# two test devices which are already next to each other rather trying to
+# sneak a new USB host on a crowded lab shelf or trying to use a laptop
+# help up in the air. Also, these are modules so not loaded unless
+# actually in use.
+CONFIG_USB_SERIAL=m
+CONFIG_USB_SERIAL_CP210X=m