ARM: dts: stm32: force b-session-valid for otg on stm32mp135f-dk board

stm32mp135f-dk board has a type-c connector to retrieve the connection
state. For now, simply force an active peripheral mode in u-boot for
flashing.

Signed-off-by: Fabrice Gasnier <fabrice.gasnier@foss.st.com>
Signed-off-by: Patrice Chotard <patrice.chotard@foss.st.com>
Reviewed-by: Patrick Delaunay <patrick.delaunay@foss.st.com>