slimbus: Add slimbus device driver unregistration function

slim_driver_unregister is added to reverse effects of
slim_driver_register. That way a slimbus device driver can be
registered when driver module is loaded, and unregistered when
the driver module is unloaded.

CRs-Fixed: 441540
Change-Id: Iae64f84c99d5027c9b55c3fde5fe71c6a550354e
Signed-off-by: Sagar Dharia <sdharia@codeaurora.org>