nvmem: core: Fix of_nvmem_cell_get() for optional cells

of_nvmem_cell_get() should return -ENOENT when a cell isn't defined,
otherwise callers can't distinguish between a missing cell and other
errors.

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