ARM: dts: stm32: add pinctrl and disabled spi5 node in stm32mp135f-dk

Add pinctrl information and a disabled spi5 node within
stm32mp135f-dk.dts in order to use the spi5 bus which is
available via the GPIO expansion pins of the STM32MP135 Discovery board.

Signed-off-by: Alain Volmat <alain.volmat@foss.st.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>