nuvoton: battery-backed ram

Add bindings and entry for the battery-backed ram in the nuvoton
npcx. This commit is an upstream port of
https://source.chromium.org/chromiumos/chromiumos/codesearch/+/main:src/platform/ec/zephyr/dts/bindings/cros_bbram/nuvoton,npcx-cros-bbram.yaml;l=1;drc=e9af813c36b7b411bf2a01cbc1b09d5fdec49b8a



Along with a bit of cleaning up and an emulator.

Signed-off-by: Yuval Peress <peress@chromium.org>