dts: arm: st: Add SPI nodes

This commits adds the "least common denominator" in the
stm32fX.dtsi files and fills the additional SPI nodes
in stm32fXYZ.dtsi files, only for the SOCs where boards
use the additional SPI peripheral.

We could add the rest SPI nodes in the stm32fXYZ.dtsi
files when we add SPI support to other boards.

Signed-off-by: Yannis Damigos <giannis.damigos@gmail.com>