mod/system_coordinator: Fix handling of final phase transition

Ensure phase processing only occurs when the next phase index is valid. Previously, a timer could be started or a phase event sent even if the current phase was the last one, depending on the phase_us value. This patch adds a check to prevent processing beyond the last valid phase, ensuring safe termination of the phase sequence.

Signed-off-by: Mohamed Omar Asaker mohamed.omarasaker@arm.com