drivers: spi: spi_ll_stm32: Add config to manage slave select

Allow the user to use software slave select instead of the
hardware pin, in order to free the related GPIO and avoid
unwanted SS triggering on the hardware pin. The default SS
is still the hardware pin.

Signed-off-by: Yaël Boutreux <yael.boutreux@st.com>
Signed-off-by: Arnaud Pouliquen <arnaud.pouliquen@st.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>