power: qpnp-smbcharger: support switching buck frequency on flash events

As per the systems team recommendation, buck switching frequency
(Fsw) needs to be increased to 1MHz whenever the flash fires. Fsw
needs to be set back to 750KHz when the flash is disabled. Add
support for this.

Currently, this is required only for PMI8996 without which there
is a potential chance for charger LS FET damage. Enable it for
PMI8996.

CRs-Fixed: 948392
Change-Id: I4d1a0bf7f97be4897873b54018df91ef61584127
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>