msm: clock: Add support for current to vdd_class

The regulator_set_optimum_mode() API is used by acpuclock-krait. As part
of the effort to rewrite this code using standard clock APIs, broaden
these standard APIs to allow configuring both regulator voltage and
current.

Change-Id: Ic6dfa5c0c18d1c3eb531727ee79e4f5be84e8a97
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>