stm32: pinmux: spi: Opt for lower power consumption

Fixes #16739. Changed the STM32's SPI MISO/MOSI configurations in order
to reduce the power consumption by approximately 20uA per pin.

According to STM32's Application Notes on GPIO configuration for
low-power consumption, the input pins should be configured with internal
pull-up or pull-down resistor. If a pin is configured as a floating
input, and there is no signal present, the Schmitt trigger randomly
toggles between the logical levels induced by the external noise, thus
increasing the consumption.

Signed-off-by: Ioannis Konstantelias <ikonstadel@gmail.com>