Slimbus: Inline a function to avoid multiple definition error

Compilation error(multiple definitons of function) happens
with slim_register_board_info function when CONFIG_SLIMBUS
flag is disabled.
In this patch slim_register_board_info function in slimbus.h
file is modified to static inline to resolve the issue.

CRs-fixed: 529039
Change-Id: I083062670139ae830dde9b0bc82c72febdf3271c
Signed-off-by: Kiran Gunda <kgunda@codeaurora.org>