ARM: dts: stm32: add jedec compatible for nor flash on stm32mp157c-ev1

This patch adds jedec compatible for spi-nor flash
on stm32mp157c-ev1 (needed with new spi-mem interface).

Signed-off-by: Ludovic Barre <ludovic.barre@st.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>