ARM: msm: Only build timer.c if required

The MSM timer is only used on MSM devices that don't have the
architected timers. Introduce a hidden Kconfig option for this
driver so that we don't build it on the platforms that don't need
it.

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