boards: arm: stm32mp157c_dk2: Add SPI support

Add SPI support for stm32mp157c_dk2 board. If SPI is selected, SPI4
(Arduino connector compatible SPI) and SPI5 (on front 2x20 GPIO
expander) will be enable by default on stm32mp157c_dk2 board.

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>