ARM: dts: s5pv210: add dummy 5V regulator for backlight on SMDKv210

Backlight is supplied by DC5V regulator.  The DTS has no PMIC node, so
just add a regulator-fixed to solve it and fix dtbs_check warning:

  s5pv210-smdkv210.dtb: backlight: 'power-supply' is a required property

Link: https://lore.kernel.org/r/20230421095721.31857-4-krzysztof.kozlowski@linaro.org


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