slim: msm: ngd: Fix incorrect casting of slimbus context-bank devices

Slimbus controller's context-bank(CB) devices are created to
facilitate IOMMU mapping for respective slimbus controllers.
Calling the suspend/resume routines on CB-devices will result in
incorrect casting of these devices to controller devices leading to
a kernel-panic.
CB-devices for slimbus don't need to do anything in suspend/resume
since power management aspects are handled by controller devices.

Change-Id: Ib83ef58c2a8e212a74bb2b215345805ef3c032bd
Signed-off-by: Sagar Dharia <sdharia@codeaurora.org>