ARM: msm: Only build clock.c on proc_comm based platforms

The functionality provided by clock.c in mach-msm is only needed
on proc_comm based platforms. Only build the file if proc_comm is
enabled. This prevents compile failures for platforms that are
part of the multi-platform kernel.

Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Olof Johansson <olof@lixom.net>