kernel: Move some macros

K_NUM_PRIORITIES and K_NUM_PRIO_BITMAPS were defined in
nano_internal.h, but used in only a handful of places.  Move to
kernel_structs.h (somewhat higher up in the hierarchy) to help with
include file cycle-breaking.  Arguably they are a better fit there
anyway.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>