ARM: dts: stm32: Add missing dm-spl props for SPI NOR on AV96

ARM: dts: stm32: Add missing dm-spl props for SPI NOR on AV96

The u-boot,dm-spl DT props are missing on AV96, hence the pinmux and
flash0 nodes are not included in the reduced SPL DT. This prevents
SPI NOR boot from working at all. Fix this by filling them in.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Patrick Delaunay <patrick.delaunay@st.com>
Cc: Patrice Chotard <patrice.chotard@st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@st.com>
Reviewed-by: Patrick Delaunay <patrick.delaunay@st.com>