clocksource: qcom: split building of legacy vs multiplatform support

The majority of the clocksource code for the Qualcomm platform is shared
between newer (multiplatform) and older platforms.  However there is a bit
of code that isn't, so only build it for the appropriate config.

Acked-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Kumar Gala <galak@codeaurora.org>