nvmem: make nvmem_add_cells() static

Now we have new api nvmem_add/del_cell_table() we do not want users to use
nvmem_add_cells() anymore. So mark it accordingly. I guess it was missed in
original cleanup patch.

This also fixes below warning:
core.c:355:5: warning: no previous prototype for 'nvmem_add_cells'
 [-Wmissing-prototypes]

Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>