dt-bindings: nvmem: make "reg" property optional

dt-bindings: nvmem: make "reg" property optional

Most NVMEM providers have cells at hardcoded addresses however there are
some exceptions. Some devices store cells layout in internal structs
using custom formats.

It's important to allow NVMEM consumers to still reference such NVMEM
cells. Making "reg" optional allows defining NVMEM cells by their names
only and using them with phandles.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>