clock-plutonium: Don't compile plutonium drivers for other targets

Clock drivers for MSMPLUTONIUM only needs to be compiled for that target.
Change compilation condition from ARCH_MSM to ARCH_MSMPLUTONIUM.

Change-Id: I1059f6078f72db98b28139a41df8cb13de3ed539
Signed-off-by: Junjie Wu <junjiew@codeaurora.org>