msm: acpuclock-krait: Use standard regulator APIs during CPU Hotplug

Voltage regulators supplying CPU cores need to be properly
notified when their underlying core goes offline. Use
regulator_disable() / regulator_enable() during this case to
notify them. Also make calls to regulator_set_voltage to
complement the regulator_disable call, since in the regulator
framework voltages are orthogonal to enable / disable.

This solves a problem on msm8974 where the ganged regulator
voltage is left higher than necessary when offlining CPU cores.

CRs-fixed: 410992
Change-Id: I1e569924133d98e75343d8b4ff24901dd2951de7
Signed-off-by: Michael Bohan <mbohan@codeaurora.org>