boards: arm: STM32 boards use DT to configure SPI

Configure SPI using DT for the following STM32 boards:

96b_neonkey
nucleo_f091rc
nucleo_f334r8
nucleo_f401re
nucleo_l432kc
nucleo_l476rg

SPI nodes in <board>.dts file are populated matching boards existing
pinmux default configuration and enabled in Kconfig.defconfig if
SPI is enabled.

For nucleo_l476rg board SPI2 and SPI3 nodes are not yet added, because
of missing pinmux defines.

Fixes #5836

Signed-off-by: Daniel Wagenknecht <wagenknecht.daniel@gmail.com>