regulator: qcom-rpm: Make it possible to specify supply

Make it possible to specify the supply of a regulator, through the
vin-supply property in dt.

Signed-off-by: Bjorn Andersson <bjorn.andersson@sonymobile.com>
Acked-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Mark Brown <broonie@kernel.org>