dt-bindings: nvmem: layouts: add fixed-layout

With the introduction of NVMEM layouts, new NVMEM content structures
should be defined as such. We should also try to convert / migrate
existing NVMEM content bindings to layouts.

This commit handles fixed NVMEM cells. So far they had to be defined
directly - as device subnodes. With this change it's allowed to put them
in the DT node named "nvmem-layout".

Having NVMEM cells in separated node is preferred as it draws a nice
line between NVMEM device and its content. It results in cleaner
bindings.

FWIW a very similar situation has happened to MTD devices and their
partitions: see commit 5d96ea42 ("dt-bindings: mtd: Clarify all
partition 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>
Message-ID: <20230611140330.154222-24-srinivas.kandagatla@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>