logging: Cleanup in log_instance

Cleanup in log_instance.h:
 - prefixing internal macros with Z_
 - adding doxygen documentation
 - using COND_CODE_1 instead of ifdefs

Additionally, added LOG_INSTANCE_PTR macro which allows to get
pointer to instance. It can be used to reuse single instance
for multiple module layers when doing instance logging.

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