spi: stm32: initialise according to `zephyr,pm-device-runtime-auto`

Don't automatically enable device runtime PM on the SPI port just
because `PM_DEVICE_RUNTIME` is enabled. Require the user to explicitly
call `pm_device_runtime_enable` on the port, or add
`zephyr,pm-device-runtime-auto` to the devicetree node.

Through the usage of `pm_device_driver_init`, the default clock control
and pinctrl handling can all be contained in `spi_stm32_pm_action`.

Signed-off-by: Jordan Yates <jordan@embeint.com>