Add support of RX timeout interrupts to limit the number of interrupts.
RX timeout is a number of bits (baud clock cycles) without
transmission seen in the receiver. One character  is used as an arbitrary
RX timeout value.
If parity is enabled, the number of bits has to include parity bit.
Signed-off-by: NGerald Baeza <gerald.baeza@st.com>
Signed-off-by: NFabrice Gasnier <fabrice.gasnier@st.com>
Signed-off-by: NErwan Le Ray <erwan.leray@st.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>