msm: timer: add api to get timer0's base

On some targets we have per-cpu timers that are accessible from either
core. Sometimes drivers need to access timer0's timer directly. Provide
the msm_timer_get_timer0_base api to abstract this for both targets with
per-core timers and targets without them.

Signed-off-by: Jeff Ohlstein <johlstei@codeaurora.org>