regulator: qcom_smd: add list_voltage callback

This patch adds support to list_voltage callback, so that consumers
like mmc core, can get information of supported voltage range.

Without this patch there is no way for mmc core to know this voltage range.

Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Mark Brown <broonie@kernel.org>