power: pm8921-charger: add pm_chg_write wrapper

A complex workaround needs to make changes in the charger
hardware block during every write to charger registers.

Fix this by adding a pm_chg_write wrapper function and
replacing all pm8xxx_writeb function calls with this
function.

Change-Id: I0c5656798444764b52f4d9d8889f831d674a1610
Signed-off-by: David Keitel <dkeitel@codeaurora.org>