drivers: spi_ll_stm32: add SUBGHZSPI support

This adds support for controlling the SUBGHZSPI NSS line in STM32WL
devices. This is a special dedicated SPI port only connected to the
radio device internally, chip select happens through a bit in the PWR
module. Adding a special dt-property to identify the port, it all gets
built out on non-WL devices.

Deduplicate the existing dts bindings in the process, and add the new
one for the special spi with the new property.

Signed-off-by: Fabio Baltieri <fabio.baltieri@gmail.com>