dt-bindings: nvmem: u-boot,env: add basic NVMEM cells

U-Boot doesn't have cells at hardcoded addresses. They are stored in
internal format. It's still important to define relevant cells in DT so
NVMEM consumers can reference them.

Update binding to allow including basic cells as NVMEM device subnodes.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Reviewed-by: Tom Rini <trini@konsulko.com>
Link: https://lore.kernel.org/r/20220703084843.21922-1-zajec5@gmail.com


Signed-off-by: Rob Herring <robh@kernel.org>