ARM: dts: exynos: add panel supply in Tiny4412

The simple panel should have a supply.  Provide a stub, as the board DTS
does not have a PMIC node, to silence warning:

  exynos4412-tiny4412.dtb: panel: 'power-supply' is a required property

Link: https://lore.kernel.org/r/20230120155404.323386-7-krzysztof.kozlowski@linaro.org


Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>