ARM: dts: stm32: relax qspi pins slew-rate for stm32mp157

Relax qspi pins slew-rate to minimize peak currents.

Fixes: 84403005 ("ARM: dts: stm32: add flash nor support on stm32mp157c eval board")

Link: https://lore.kernel.org/r/20191025130122.11407-1-alexandre.torgue@st.com


Signed-off-by: Patrice Chotard <patrice.chotard@st.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>
Signed-off-by: Olof Johansson <olof@lixom.net>