dts: mec172x: move the uart device node off espi

Move the two UART nodes so that they are under "soc" rather than "espi",
leave only xec-espi-host-dev nodes there.

The UART device can be used indepdently by the driver uart_mchp_xec.c
and it's normally initialized before before the espi one.

Moving the device node up a level so this does not trigger a false
positive on the build time priority checking.

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>