msm: msm_bus: Modify bus makefile layout

Modify the bus makefile layout so as not to require adding a new target
ifdef to compile the same bus id object file. This will also help in
binary compatibility between targets.

CRs-Fixed: 546650
Change-Id: Icec46a36bcbb88652b02cd2e2dfaf25c99251ef6
Signed-off-by: Girish Mahadevan <girishm@codeaurora.org>