servo_micro: add UART3 routing in servod

This provides a driver and control for switching the usbpd uart
to spi lines (samus) or jtag lines (glados), allowing pd controller
interaction through servod.

BUG=chromium:660922
TEST=can access PD uart on samus, chell after usbpd_uart_routing:glados/samus

Change-Id: Ie41d5b045a15bf640ee107f4f014b219c08b20a6
Reviewed-on: https://chromium-review.googlesource.com/407539
Commit-Ready: Nick Sanders <nsanders@chromium.org>
Tested-by: Nick Sanders <nsanders@chromium.org>
Reviewed-by: Kevin Cheng <kevcheng@chromium.org>
3 files changed