power: smb1351-charger: Update smb1351 switching frequency

The default switching frequency of 750kHz is causing spur
on some devices, so increase the switching frequency to 1.5MHz.

CRs-Fixed: 1018750
Change-Id: I09da751b414d09729eb07c36ff1e9b667237833b
Signed-off-by: Devesh Jhunjhunwala <deveshj@codeaurora.org>