spi: stm32: use SoC specific compatible

This patch replaces st,stm32-spi compatible with st,stm32h7-spi SoC
specific compatible and updates the example accondingly.

Signed-off-by: Amelie Delaunay <amelie.delaunay@st.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Mark Brown <broonie@kernel.org>