ARM: 9261/1: amba: Drop redundant assignments of the system PM callbacks

If the system PM callbacks haven't been assigned, the PM core falls back to
invoke the corresponding the pm_generic_* helpers for the device. Let's
rely on this behaviour and drop the redundant assignments.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>