Relax qspi pins slew-rate to minimize peak currents.

Fixes: 84403005 ("ARM: dts: stm32: add flash nor support on stm32mp157c eval board")
Signed-off-by: Patrice Chotard <patrice.chotard@st.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>