power: qpnp-smb2: Specify the min/max charger switching frequency

Add a device tree property to override the minimum and maximum
charger buck/boost switching frequency.

Change-Id: I6b827157c23a389edf2daa412e6825b6ce07a886
Signed-off-by: Anirudh Ghayal <aghayal@codeaurora.org>