drivers: spi: make SPI dt-spec macros compatible with C++

As of today it is not possible to use SPI dt-spec macros in C++,
something known and documented. The main reason is because `cs` property
is initialized using a compound literal, something not supported in C++.
This PR takes another approach, that is to not make `cs` a pointer but a
struct member. This way, we can perform a regular initialization, at the
cost of using extra memory for unused delay/pin/flags if `cs` is not
used.

Fixes #56572

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>