power_supply: add property to reflect switcher frequency

Modern battery chargers use a switching buck whose frequency
can be adjusted. Create a property to read/update it.

CRs-Fixed: 1048242
Change-Id: I7611e6b697222427303df47f837716005e013443
Signed-off-by: Harry Yang <harryy@codeaurora.org>
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>