kernel: Apply const to k_pipe_put() parameter

The pointer parameter 'data' in the function 'k_pipe_put()' ought to
use the const modifier as the contents of the buffer to which it
points never change. Internally, that const modifier is dropped as
both 'k_pipe_get()' and 'k_pipe_put()' share common code for copying
data; however 'k_pipe_put()' never takes a path that modifies those
contents.

Signed-off-by: Peter Mitsis <peter.mitsis@intel.com>