dt-bindings: nvmem: convert base example to use NVMEM fixed cells layout

With support for the "fixed-layout" binding it's possible and preferred
now to define fixed NVMEM cells in the layout node. Do that for the
example binding.

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: <20230611140330.154222-25-srinivas.kandagatla@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>