dt-bindings: nvmem: use base meta-schema for consumer schema

Common consumer schemas need to use the base.yaml meta-schema because
they need to define different constraints (e.g. the type) from what
users of the common schema need to define (e.g. how many entries).

Cc: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Rob Herring <robh@kernel.org>
Reviewed-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Link: https://lore.kernel.org/r/20210316195111.3531755-1-robh@kernel.org