pmic:pfuze implement pmic_mode_init

This patch is to implement pmic_mode_init function, and add prototype
in header file.

This function is to set switching mode for pmic buck regulators to
improve system efficiency.

Mode:
OFF: The regulator is switched off and the output voltage is discharged.
PFM: In this mode, the regulator is always in PFM mode, which
     is useful at light loads for optimized efficiency.
PWM: In this mode, the regulator is always in PWM mode operation
     regardless of load conditions.
APS: In this mode, the regulator moves automatically between
     pulse skipping mode and PWM mode depending on load conditions.

Signed-off-by: Peng Fan <Peng.Fan@freescale.com>
Acked-by: Przemyslaw Marczak <p.marczak@samsung.com>