msm: timer: Add a stub for msm_timer_get_timer0_base

This function is not defined if native ARM timers are used,
leading to compilation errors in shared restart core.

Change-Id: Idbb393f9e5f884d31088be211a5456c1d76b5d02
Signed-off-by: Stepan Moskovchenko <stepanm@codeaurora.org>