nvmem: Change to unified property interface

Change from using device tree (Open Firmware) APIs to the unified
'fwnode' interface.

Change of_nvmem_cell_get() to fwnode_nvmem_cell_get(), and add a
wrapper for of_nvmem_cell_get().

Change of_nvmem_device_get() to fwnode_nvmem_device_get(). There
are no known accessors to the OF interface, so no need for a wrapper.

The first version of this patch incorrectly had a wrapper for
of_nvmem_device_get(), even though the comments about the patch
not needing this were correct.

The second version of this patch had an incorrect return type for
of_nvmem_device_get().