power: pm8921-charger: add PM8038 buck clock control address

The CHG_BUCK_CLOCK_CTRL register address changed in PM8038.

Add this address and make sure the correct buck clock frequency
is set in the hardware initialization.

Change-Id: I96b47c8603a291c41c5769fab9483d83fefb6084
Signed-off-by: David Keitel <dkeitel@codeaurora.org>