cm: charger: Add support for double backlight displays

 * Some devices using AMS AS36xx LEDs controllers
    are using 2 backlight paths to light the display,
    mostly on Sony Huashan and Sony Blue board devices

 * Although most of the displays have interlaced backlights
    and the half dark path can be hard to see, other displays
    have a top / bottom path that make half the screen on / off

 * Control the backlight level of both path if the device does have
    BACKLIGHT_PATH set, and if present, SECONDARY_BACKLIGHT_PATH

 * Leave the HEALTD_BACKLIGHT_LEVEL config accessible
    to override the default 100 / 255 brightness used in healthd

Change-Id: If774c3e66acedddf7ba676581e7c88b7e83a66b6
Signed-off-by: Adrian DC <radian.dc@gmail.com>