drivers: pinmux: stm32wb: use a pull-down for SPI SCK pin

Like stm32 L4 does,
when the STM32WBx SoC goes into STOP mode, the SPI device is disabled.
This cause the pins to not be drived anymore (i.e. they are floating)
except through their pull-up or pull-down.

From the logical point of view, the NSS pin is held high by a pull-up
so it's not a problem if the other pins are floating. However those pins
are floating input for the slaves, which increase their power
consumption.

The solution is to hold the state of the pins through a pull-up or a
pull-down. This is already done for the NSS and MOSI pins, but not for
SCK. Fix that by using pull-down on the SCK pin the same way it is
already done for the MOSI pin.

Signed-off-by: Francois Ramu <francois.ramu@st.com>