dt-bindings: nvmem: brcm,nvram: add basic NVMEM cells

dt-bindings: nvmem: brcm,nvram: add basic NVMEM cells

NVRAM doesn't have cells at hardcoded addresses. They are stored in
internal struct (custom & dynamic 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: Rob Herring <robh@kernel.org>
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>