tps65090: Throttle PMU interrupt instead of charger task

This is a prepartory work for integrating USB port control into PMU
task.

Currently TPS65090 charger task is throttled to only waken by event once
per 500 ms. This in a way makes it hard to integrate other functionality
into this task. This CL moves the throttling mechanism to interrupt
handler so as to provide better control of when to throttle the
interrupt event.

BUG=chrome-os-partner:14319
TEST=Build success and boot on spring.
BRANCH=none

Change-Id: I72e63180442b379a379e1a87c10ef62395434872
Signed-off-by: Vic Yang <victoryang@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/40189
Reviewed-by: Rong Chang <rongchang@chromium.org>

include/pmu_tpschrome.h

@@ -224,5 +224,8 @@ int board_get_ac(void);
  */
 void board_hard_reset(void);
 
+/* Wake TPS65090 charger task, but throttled to at most one call per tick. */
+void pmu_task_throttled_wake(void);
+
 #endif /* __CROS_EC_TPSCHROME_H */