power: core: add power supply APIs

Add two APIs, 1) set_current_limit API and 2) set_charging_by.
set_current_limit api is used for external control, such as for USB
driver to tell a charger driver the maximum current to draw.
set_charging_by is used to change the state of the charger.

Change-Id: I147eab37836b54627ca1458167eb117ba3eacd7a
Signed-off-by: Willie Ruan <wruan@codeaurora.org>