drivers: esp32: Add minimal UART driver based on ROM routines

This is a minimal driver enabling console output during the port
bringup.  While the driver works, only one of the three UART devices
are supported, and there isn't any way to change any parameters or
use interrupts.  This will most likely be superceded by a proper
driver after the port has matured.

Signed-off-by: Leandro Pereira <leandro.pereira@intel.com>