arm64: dts: qcom: db820c: Use regulator names from schematics

Update the regulator names in db820c.dtsi to use the names from the
schematics, instead of the made up genric names.

Acked-by: Vinod Koul <vkoul@kernel.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>