Make uart_init and uart_deinit internal functions. And make
uart_set_backend/uart_release_backend cover uart_init/uart_deinit
function.
This will make mevent teardown callback adding easier for uart_core.
Tracked-On: #1877
Signed-off-by: Yin Fengwei <fengwei.yin@intel.com>
Acked-by: Yu Wang <yu1.wang@intel.com>