dt-bindings: power: reset: $ref reboot-mode in nvmem-reboot-mode

nvmem-reboot-mode.yaml should $ref: reboot-mode.yaml, but instead
rewrites the properties. Update so it $refs instead.

Signed-off-by: Elliot Berman <quic_eberman@quicinc.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Mukesh Ojha <quic_mojha@quicinc.com>
Link: https://lore.kernel.org/r/20231031-ref-nvmem-reboot-mode-v1-1-c1af9070ce52@quicinc.com


Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>