logging: Add empty Z_LOG_MSG2_SIMPLE_CREATE version

Add empty Z_LOG_MSG2_SIMPLE_CREATE when CONFIG_LOG_SPEED=n. It
allows to reduce compilation time by removing code at preprocessor
stage. Previously functionality was the same, compiler was removing
unused code but because logging is widely use that had a visible
impact on compile time.

Signed-off-by: Krzysztof Chruscinski <krzysztof.chruscinski@nordicsemi.no>