spi: stm32_qspi: Add st, stm32f469-qspi compatible string

Add "st,stm32f469-qspi" compatible which is used on kernel side.
This will be necessary when DT will be synchronised from kernel.

Signed-off-by: Christophe Kerello <christophe.kerello@st.com>
Signed-off-by: Patrice Chotard <patrice.chotard@st.com>
Reviewed-by: Jagan Teki <jagan@openedev.com>