qcom-charger: set optimal buck switching frequency

Reduce the power dissipation impact by choosing the optimal
switching frequency per input voltage.

The optimal switching frequences are chosen as follow,
	5V only = 600kHz
	6-8V QC3 = 800kHz
	9V QC2 or vanilla PD = 1.0MHz
	12V QC2 or PD adapter = 1.2MHz

CRs-Fixed: 1048242
Change-Id: I2ef9f3e122b39682a743334668fcd0aeb085e147
Signed-off-by: Harry Yang <harryy@codeaurora.org>
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>