ARM: dts: qcom: pma8084: fix vadc channel node names

The spmi-vadc bindings require the '@' in the node.

Additionally change the node name to adc-chan which both makes it a
generic node name and also removes the underscore from it.

Signed-off-by: Luca Weiss <luca@z3ntu.xyz>
Signed-off-by: Bjorn Andersson <andersson@kernel.org>
Link: https://lore.kernel.org/r/20221031181022.947412-1-luca@z3ntu.xyz