arm64: dts: qcom: add lable for smd rpm regulators

This patch adds label to smd rpm regulators so that the board level file
can use the label directly to populate the regulators, rather than
having deep nesting.

Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Andy Gross <andy.gross@linaro.org>