ARM: dts: qcom: pm8226: Add VADC node

Add a node for the voltage ADC (VADC) found in PM8226.

Signed-off-by: Rayyan Ansari <rayyan@ansari.sh>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Link: https://lore.kernel.org/r/20220209163841.7360-1-rayyan@ansari.sh