UPSTREAM: nvmem: use kref

Use kref for reference counting. Use an approach similar to the one
seen in the common clock subsystem: don't actually destroy the nvmem
device until the last user puts it. This way we can get rid of the
users check from nvmem_unregister().

Bug: 154188491
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
(cherry picked from commit c1de7f43bd84271e438a4f2dadc4ba9a353baaea)
Signed-off-by: Will McVicker <willmcvicker@google.com>
Change-Id: I4c05a0614255d72163b79526324e41dd4b0100ef