regulator: qcom_spmi: Add support for get_mode/set_mode on switches

The voltage switches support mode switching, so add support for
these ops to those types of regulators.

Cc: Bjorn Andersson <bjorn.andersson@linaro.org>
Fixes: e92a4047

 ("regulator: Add QCOM SPMI regulator driver")
Signed-off-by: Stephen Boyd <stephen.boyd@linaro.org>
Signed-off-by: Mark Brown <broonie@kernel.org>