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.

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

Change-Id: Ie41d5b045a15bf640ee107f4f014b219c08b20a6
Commit-Ready: Nick Sanders <>
Tested-by: Nick Sanders <>
Reviewed-by: Kevin Cheng <>
3 files changed