nvmem: brcm_nvram: find Device Tree nodes for NVMEM cells

DT binding for Broadcom's NVRAM supports specifying NVMEM cells as NVMEM
device (provider) subnodes. Look for such subnodes when collecing NVMEM
cells. This allows NVMEM consumers to use NVRAM variables.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Link: https://lore.kernel.org/r/20220429162701.2222-3-srinivas.kandagatla@linaro.org


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