HAL_ChibiOS: switch to minimum scheduling delta of 10us

this avoids an issue with the ChibiOS 20.3 virtual timer
implementation that can result in a scheduling slip equal to the
system timer period.

Andy has been suggesting this change for a while. I resisted it as I
thought it would impact on soft-serial parsing, but it turns out it
doesn't.

Fixes issue #18383

libraries/AP_HAL_ChibiOS/Scheduler.cpp

@@ -142,7 +142,7 @@ void Scheduler::delay_microseconds(uint16_t usec)
         // calling with ticks == 0 causes a hard fault on ChibiOS
         ticks = 1;
     }
-    chThdSleep(ticks); //Suspends Thread for desired microseconds
+    chThdSleep(MAX(ticks,CH_CFG_ST_TIMEDELTA)); //Suspends Thread for desired microseconds
 }
 
 /*

libraries/AP_HAL_ChibiOS/hwdef/common/chconf.h

@@ -119,7 +119,7 @@ extern "C" {
  *          this value.
  */
 #ifndef CH_CFG_ST_TIMEDELTA
-#define CH_CFG_ST_TIMEDELTA                 2
+#define CH_CFG_ST_TIMEDELTA                 10
 #endif
 
 /*