armcm_timer: Use a static instruction count for TIMER_MIN_TRY_TICKS

Change TIMER_MIN_TRY_TICKS from 2us to 90 instructions.

On newer chips 2us is a large amount of time - for example on the
520Mhz stm32h723 it would be 1040 instructions.  Using a large time
can result in "busy waiting" in the irq handler when the cpu may be
better spent running tasks.

The armcm_timer.c code is used on most ARM cortex-M chips and on all
of these chips the SysTick timer should be tied directly to the
instruction counter.  This change should be safe because it should not
take more than 90 instructions to reschedule the timer on any of these
chips.  Also, all of these chips should be able to exit the irq
handler and reenter it in less than 90 instructions allowing more time
for tasks to run if the next timer is more than 90 timer ticks in the
future.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

src/generic/armcm_timer.c

@@ -114,7 +114,7 @@ DECL_INIT(timer_init);
 static uint32_t timer_repeat_until;
 #define TIMER_REPEAT_TICKS timer_from_us(100)
 
-#define TIMER_MIN_TRY_TICKS timer_from_us(2)
+#define TIMER_MIN_TRY_TICKS 90
 #define TIMER_DEFER_REPEAT_TICKS timer_from_us(5)
 
 // Invoke timers