dt-bindings: power: mediatek: Add another nested power-domain layer

The MT8188 SoC has a more in-depth power-domain tree, and the
CHECK_DTBS=y check could fail because the current MediaTek power
dt-binding is insufficient to cover its CAM_SUBA and CAM_SUBB
sub-domains.

Add one more nested power-domain layer to pass the check.

Acked-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: Fei Shao <fshao@chromium.org>
Message-ID: <20241001113052.3124869-2-fshao@chromium.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>