nvmem: core: add single sysfs group

The sysfs core provides a function to easily register a single group.
Use it and remove the now unnecessary nvmem_cells_groups array.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Link: https://lore.kernel.org/r/20240705074852.423202-10-srinivas.kandagatla@linaro.org


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>