dt-bindings: nvmem: Add missing 'reg' property

dt-bindings: nvmem: Add missing 'reg' property

With 'unevaluatedProperties' support implemented, the following warnings
are generated in the nvmem examples:

Documentation/devicetree/bindings/nvmem/st,stm32-romem.example.dt.yaml: efuse@1fff7800: Unevaluated properties are not allowed ('reg' was unexpected)
Documentation/devicetree/bindings/nvmem/rmem.example.dt.yaml: nvram@10000000: Unevaluated properties are not allowed ('reg' was unexpected)
Documentation/devicetree/bindings/nvmem/brcm,nvram.example.dt.yaml: nvram@1eff0000: Unevaluated properties are not allowed ('reg' was unexpected)

Add the missing 'reg' property definition.

Cc: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Cc: Maxime Coquelin <mcoquelin.stm32@gmail.com>
Cc: Alexandre Torgue <alexandre.torgue@foss.st.com>
Cc: Rafał Miłecki <rafal@milecki.pl>
Cc: Saenz Julienne <nsaenzjulienne@suse.de>
Cc: Fabrice Gasnier <fabrice.gasnier@foss.st.com>
Cc: linux-stm32@st-md-mailman.stormreply.com
Cc: linux-arm-kernel@lists.infradead.org
Signed-off-by: Rob Herring <robh@kernel.org>
Reviewed-by: Fabrice Gasnier <fabrice.gasnier@foss.st.com>
Reviewed-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>