regulator: implement basic reference counter

Some devices share a regulator supply, when the first one will request
regulator disable, the second device will have it's supply cut off before
graciously shutting down. Hence there will be timeouts and other failed
operations.
Implement a reference counter mechanism similar with what is done in
Linux, to keep track of enable and disable requests, and only disable the
regulator when the last of the consumers has requested shutdown.

Signed-off-by: Eugen Hristev <eugen.hristev@collabora.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>