ARM: dts: stm32: Use kernel qspi compatible string for stm32f469-disco-uboot.dtsi

For STM32 QSPI driver, "st,stm32-qspi" compatible string was first
introduced in U-boot. But later in kernel side, "st,stm32f469-qspi"
was used.
To simplify, align U-boot QSPI compatible string with kernel one.

Signed-off-by: Patrice Chotard <patrice.chotard@st.com>