serial: liteuart: move function definitions

Move definitions for liteuart_[stop|start]_tx(), liteuart_stop_rx(),
and liteuart_putchar() to a more convenient location in preparation
for adding IRQ support. This patch contains no functional changes.

Signed-off-by: Gabriel Somlo <gsomlo@gmail.com>
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>
Reviewed-by: Jiri Slaby <jirislaby@kernel.org>
Link: https://lore.kernel.org/r/20221123130500.1030189-12-gsomlo@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>