nvmem: u-boot-env: find Device Tree nodes for NVMEM cells

DT binding allows specifying NVMEM cells as NVMEM device (provider)
subnodes. Looks for such subnodes when building NVMEM cells.

This allows NVMEM consumers to use U-Boot environment variables.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Link: https://lore.kernel.org/r/20220916122100.170016-11-srinivas.kandagatla@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>