api: move a devicetree.h layering violation to drivers/spi.h

The DT_SPI_DEV_CS_GPIOS_DT_SPEC_GET macro belongs in drivers/spi.h,
not devicetree.h.

It is creating a struct gpio_dt_spec, but the devicetree.h API does
not (other than in this case) and should not depend on structures that
are defined in the GPIO API. This is because the GPIO API already
depends on the devicetree.h API, so making a dependency in the reverse
direction creates a needless circular dependency.

This macro should have been added to the drivers/spi.h API from the
beginning. Move it there under a new name, SPI_CS_GPIOS_DT_SPEC_GET.

We haven't created a Zephyr release with
DT_SPI_DEV_CS_GPIOS_DT_SPEC_GET in it, so there is no need to go
through the stable API change process for devicetree.h to deprecate
and eventually remove it. We can just remove it directly.

Fixes: #42149
Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>