serial: stm32: add fifo support

This patch adds fifo mode support for rx and tx.

A fifo configuration is set in each port structure.
Add has_fifo flag to usart configuration to use fifo only when possible.

Signed-off-by: Gerald Baeza <gerald.baeza@st.com>
Signed-off-by: Bich Hemon <bich.hemon@st.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>